v0.48.0: web mixes upload to a web server (rsync over ssh), and the player shuffles

Export Web Mix gets an 'Upload to web server' box: the mix is built in a
cache staging folder and mirrored with rsync --delete into <path>/<slug>/
on the host set in Preferences → Integrations → Web Server (config.json,
this machine only). An ssh check asks before replacing an existing mix;
a status window follows check, export and upload. The web player gains a
shuffle button (and the S key).

CLAUDE.md

@@ -644,6 +644,23 @@ persistence) → GUI (Qt widgets that read the manager and connect to its signal
   untouched export still renders the original `#8c764a`. Templates are
   registered in `setup.py` via `package_data`; loaded with
   `importlib.resources` so an installed copy finds them.
+  **Upload to web server** (Round 81, `export/upload.py`): a ticked box in
+  the web-mix dialog builds the mix in `$XDG_CACHE_HOME/lintunes/webmix/
+  <slug>/` (no folder picker) and mirrors it with `rsync -a --checksum
+  --delete` over ssh to `<host>:<path>/<slug>/`. rsync, not scp: scp re-sends
+  everything and never deletes, so a song taken out of the playlist would
+  stay online forever. Host and path live in **`config.json`** only (the
+  host is usually this machine's `~/.ssh/config` alias, and trav's server
+  must never appear in the repo or the synced share — tests use
+  `example-host`). `--delete` only ever reaches the slug folder
+  (`remote_dir` validates the slug), staging is wiped first
+  (`reset_staging`, guarded to stay inside the staging root) so a removed
+  song can't survive the mirror, and ssh runs with `BatchMode=yes`, since
+  nobody can answer a prompt. Steps: `ServerCheckWorker` (`ssh test -d`; an
+  existing folder asks before replacing it), `ExportWorker`, then
+  `UploadWorker`, all shown in `gui/web_upload_progress.py`. The web
+  player has a **shuffle** button (and the S key): a shuffled walk that starts at
+  the current song and plays every song once.
 
 - **`lintunes/cassette/`** — friend library sharing (Round 63+; the spec is
   `Cassette friend library.md`). LinTunes never talks to a friend: it drives