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.

lintunes/__init__.py

@@ -1,3 +1,3 @@
 """LinTunes — iTunes-style music library manager and player for Linux."""
 
-__version__ = "0.25.0"
+__version__ = "0.26.0"