README: new features, and a Cassette guide for first-timers

Adds the features shipped since the list was written (Identify Track,
Import from URL, andTunes, art sources, New Tracks, merge improvements,
Cassette) without touching the existing lines, and a step-by-step guide
at the end: installing Syncthing, inviting a friend, choosing what to
share, cassetting songs, following playlists and troubleshooting. Icons
and screenshots in docs/cassette/ are rendered from the app's own
painting code against the synthetic dev library.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

README.md

@@ -29,12 +29,25 @@ I have been a mac user for ~34 years. I gave up daily driving mac os in 2021. Af
   `Artist/Album/` tree.
 - **Undo/redo everything** (Ctrl+Z / Ctrl+Shift+Z) — edits, deletes, reorders.
 - Deleting a track sends the file to your desktop **Trash**, never `rm`.
+- **Identify Track…** listens to a song (Chromaprint + AcoustID) and proposes
+  its real title, artist, album and *original* release year — and when the
+  internet has never heard of it, reads the answer out of the filename.
+- **Import from URL…** (File menu) — paste a link and the song is downloaded
+  (yt-dlp), filed, and handed straight to Identify Track.
+- Album art is searched on both iTunes and **Deezer**, can be pasted onto a
+  whole album's worth of tracks at once, and clicking the blank cover in the
+  sidebar goes looking for it.
+- Right-click any song to **look it up on YouTube**.
+- Every song you add is stamped with the day you added it.
 
 **Playlists**
 - Regular playlists, **folders**, and **smart playlists** (auto-updating, with a
   full rule editor).
 - Drag and drop to reorder or insert at a spot; duplicate-aware adds;
   copy/cut/paste tracks between playlists.
+- Click a selected playlist's name to rename it.
+- New libraries start with a **New Tracks** smart playlist (everything added in
+  the last 3 months).
 
 **Playing**
 - mp3, m4a, flac, aac, wav, aiff.
@@ -43,18 +56,34 @@ I have been a mac user for ~34 years. I gave up daily driving mac os in 2021. Af
 - Live **spectrum visualizer** — click it to cycle bright / dim / off.
 - last.fm scrobbling
 - Theming: highlight color, UI scale, and custom colors
+- A soft emboss on every button that follows your custom colors.
 
 **Sending music elsewhere**
 - **Chromecast**
 - **Export a playlist** (File ▸ Export Playlist…) — either a folder of
   `Artist - Title.mp3` files plus an `.m3u`, or a self-contained `index.html` you can upload anywhere.
 - **Sync a playlist to a Rabbit R1** over MTP, `.m3u` included.
+- **andTunes**, a small music player for the Rabbit R1 that LinTunes installs
+  and fills from the Connections menu — and the plays you rack up on the Rabbit
+  come back into your library.
 
 **Living on more than one machine**
 - The whole library is **plain JSON**, designed to be synced with
   [Syncthing](https://syncthing.net). Sync conflicts merge themselves on
   startup.
 - The version number in the status bar checks for updates (via git) and you can click it to update to the latest version.
+- Play counts can never conflict (each machine keeps its own tally), a song you
+  remove on one machine stays removed, and the merge report only speaks up when
+  something you'd care about happened.
+
+**Sharing with friends (Cassette)**
+- Share parts of your library with a friend — one pasted code connects you,
+  over [Syncthing](https://syncthing.net), with no server anywhere.
+- You choose exactly what each friend can see: the whole library, or just
+  certain artists, albums, songs and playlists.
+- Browse a friend's library and press the little cassette buttons on anything
+  you'd like; those songs arrive in your library by themselves.
+- Follow a friend's playlist and play it right from your own sidebar.
 
 
 
@@ -248,3 +277,177 @@ python3 -m pytest tests/
 ```
 
 `spec.md` is the original design brief; `tasks*.md` track what's built.
+
+## Sharing music with friends (Cassette)
+
+Cassette lets you and a friend peek at each other's libraries and ask for
+songs. There's no server and no account: your two computers talk directly
+through a free program called **Syncthing**, and LinTunes just tells Syncthing
+what to do. Neither of you can change or delete anything on the other's
+computer.
+
+You'll both need LinTunes and Syncthing. You **don't** need to be online at the
+same time — things simply finish the next time both computers are on.
+
+### 1. Install and start Syncthing (once)
+
+Open a terminal (see *Never used a terminal?* above) and paste the line for
+your distro:
+
+```sh
+sudo apt install syncthing        # Debian / Ubuntu / Mint
+sudo dnf install syncthing        # Fedora
+```
+
+Then start it, and have it start by itself every time you log in:
+
+```sh
+systemctl --user enable --now syncthing
+```
+
+To check it's running, open <http://127.0.0.1:8384> in your web browser —
+you should see Syncthing's page. You don't have to do anything there; the
+default settings (including "global discovery" and "relaying", which let two
+computers find each other across the internet) are exactly right.
+
+> **Already using Syncthing to keep your library on two of your own
+> computers?** Great — nothing changes. Only one of your computers handles
+> friends; LinTunes asks which the first time (see below), and songs it brings
+> in reach your other computers the usual way.
+
+### 2. Connect with a friend
+
+One of you **invites**, the other **pastes**. It doesn't matter who.
+
+**If you're inviting:** in LinTunes choose **Connections ▸ Share Library with a
+Friend…**. The very first time, LinTunes asks *"Use this computer for sharing
+with friends?"* — say **Yes**. You'll get an **invite code**: a long line
+starting with `LTC1.` Press **Copy Code** and send it to your friend however you
+like (text, email, chat). Each code works for **one** friend, once.
+
+**If you were sent a code:** choose **Connections ▸ Add Friend Library…**,
+paste the whole code into the box, and press **Connect**.
+
+That's it. When the two computers have found each other, you'll see
+**"*Name* is now connected."** at the bottom of the window. This can take a
+minute — or, if the other computer is off, however long until it's back on.
+
+You can see how it's going any time in **Connections ▸ Sync Settings…**: pick
+your friend in the list on the left. Invites nobody has used yet are listed on
+the **General** page, with a **Cancel Invite** button each.
+
+### 3. Choose what your friend can see
+
+**Nothing is shared until you say so.** In **Connections ▸ Sync Settings…**,
+click your friend's name on the left. Below their connection status are two
+sections:
+
+- **Library** — tick the artists, albums or songs your friend may ask for, or
+  tick **Share whole library** for everything (including songs you add later).
+- **Playlists** — tick the playlists to share, or **Share all playlists**.
+  A shared playlist brings its songs along: they show up ticked and greyed in
+  the Library section (hover one to see which playlist is holding it there).
+
+**Select All** and **Select None** do what they say — and if you click one by
+accident, just press **Cancel**: nothing is saved until you press **OK**.
+
+Want everyone to see the same thing? On the **General** page, tick **Share the
+same selection with all friends** and choose there instead.
+
+Sharing something doesn't send it. Your friend only receives what they ask
+for.
+
+### 4. Browse a friend's library and ask for songs
+
+Once you have a friend, a small **▾** appears on the right end of the
+**Library** button:
+
+<img src="docs/cassette/library-selector.png" alt="The Library button with the ▾ friend selector on its right" width="216">
+
+Click the **▾** and pick your friend. The button turns light purple and shows
+their library's name — you're now looking at what they've shared with you:
+
+<img src="docs/cassette/friend-library-button.png" alt="The Library button in friend mode, purple, reading Sam's Library" width="216">
+
+![Browsing a friend's library](docs/cassette/friend-mode.png)
+
+It works just like your own library, except nothing plays (the songs aren't on
+your computer yet). Instead, every artist, album and song has a little
+**cassette button**:
+
+| Button | What it means |
+|---|---|
+| <img src="docs/cassette/cassette-off.png" width="28" alt="cassette button, not pressed"> | Not asked for. Click it to ask your friend for this. |
+| <img src="docs/cassette/cassette-on.png" width="28" alt="cassette button, pressed"> | Asked for — this song is on its way to you. Click again to change your mind. |
+| <img src="docs/cassette/cassette-partial.png" width="28" alt="cassette button, partly pressed"> | Some of this artist's or album's songs are asked for. Click to ask for all of them. |
+
+Pressing the button on an **artist** or **album** asks for every song under it.
+
+Along the bottom of the window:
+
+- **Hide tracks I have** hides songs you already own. Songs that are *probably*
+  the same as one of yours (same artist and title, different album or length)
+  stay visible in light grey so you can decide.
+- The **size** of what you've asked for, next to your free space. It turns
+  **orange** if it would leave so little space that Syncthing pauses, and
+  **red** if it won't fit at all.
+- **Save and Close** sends your list to your friend and takes you back to your
+  own library. **Cancel** forgets the changes. (Leaving any other way with
+  changes asks *"Save changes?"* first.)
+
+The songs you asked for arrive in your library **by themselves**, filed by
+artist and album like anything you'd add yourself, with *"3 songs arrived from
+Sam."* at the bottom of the window. They come with their names, album info and
+artwork — but not your friend's play counts or ratings.
+
+### 5. Follow a friend's playlist
+
+In a friend's library, their shared playlists are listed on the left, each
+with a UFO. Click a playlist's **UFO** to follow it:
+
+| Icon | What it means |
+|---|---|
+| <img src="docs/cassette/ufo-not-followed.png" width="28" alt="grey outline UFO"> | Not following. Click the UFO to follow this playlist. |
+| <img src="docs/cassette/ufo-followed.png" width="28" alt="purple UFO"> | Following. Click again to stop. |
+
+Press **Save and Close**, and the playlist appears in **your** sidebar with a
+purple UFO, and plays like any other playlist:
+
+![A followed playlist in the sidebar](docs/cassette/followed-playlist.png)
+
+- Songs that are still on their way show in grey; they become playable as they
+  arrive. The bottom of the window says when your friend's folder last synced.
+- A followed playlist's songs are **kept separate from your library** — you get
+  to listen without them filling up your collection. Like one? Right-click the
+  column headings, turn on **Cassette**, and press its cassette button to add
+  it to your library.
+- Right-click a followed playlist in the sidebar and choose **Unfollow** to let
+  it go (its songs are cleaned up afterwards).
+
+Two choices per friend, in **Sync Settings** on their page:
+
+- Show followed playlists **mixed in with yours**, or grouped **in a folder
+  with their name**.
+- **Keep their songs separate** (the default), or **add them to your library**
+  automatically.
+
+### If something isn't working
+
+Sync Settings always says what's going on in plain words and what to do next.
+The common ones:
+
+- **"Syncthing isn't running"** — run `systemctl --user start syncthing`, then
+  press **Retry** on the General page.
+- **"Can't reach Syncthing's API"** — press **Read from Syncthing** on the
+  General page. (Or copy the key from Syncthing's page at
+  <http://127.0.0.1:8384>: **Actions ▸ Settings ▸ General ▸ API Key**.)
+- **"Waiting for *Name*'s computer"** for a long time — both computers need to
+  be on with Syncthing running, and your friend needs to have opened LinTunes
+  once since you pasted the code. If it never connects, the code may already
+  have been used: ask for a new one.
+- **"That code isn't valid"** — part of the code got lost when it was copied.
+  It's one long line starting with `LTC1.`; ask your friend to send it again.
+- **"This invite has already been used"** — codes work once. Ask for a new one.
+
+To stop sharing with someone, open their page in Sync Settings and press
+**Remove Friend…**. Songs you already brought into your library stay.