v0.26.0: Cassette footing — Syncthing client, host machine, Sync Settings

First step of friend library sharing (Cassette):

- cassette/syncthing_api.py: a typed wrapper over the *local* Syncthing's
  REST API. Key and address come from Syncthing's config.xml (config.json
  may override); every failure is a SyncthingError naming the step, what
  Syncthing said, and whether it's not running / a bad key / a refusal.
- cassette/state.py: friends, invites and share selections, kept in the
  machine-local ~/.local/share/lintunes/cassette, never the synced data dir.
- cassette/host.py: one machine runs Cassette. config.json flags it,
  preferences.json names it, and the other machine greys the menu group
  out and says where friend sharing lives.
- Connections ▸ Sync Settings…: VLC-style list + page, editing a copy
  that only OK writes. General shows Syncthing's status with a next step
  for every failure, the display name and an avatar picker.

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

lintunes/preferences.py

@@ -41,6 +41,9 @@ DEFAULTS = {
         "playlist_ids": [],
         "copy_art": True,
     },
+    # Which machine runs friend sharing (Cassette): {machine_id, hostname}.
+    # Synced so the other machine can name it; see lintunes/cassette/host.py.
+    "cassette_host": None,
 }