v0.27.0: Cassette invites — one paste connects two friends

- Connections ▸ Share Library with a Friend… makes a one-time invite code
  (device id, display name, token) and the inviter's send-only folder.
- Connections ▸ Add Friend Library… takes the pasted code: adds the
  inviter, shares our folder, and accepts theirs receive-only when it's
  offered back.
- Syncthing hides an unknown device's folders (verified on 1.30), so the
  inviter probes a new knock only while an invite is open: added with
  nothing shared, completed on the right token, otherwise removed and
  never probed again. Used and cancelled codes go nowhere.
- Sync Settings lists open invites (Cancel Invite) and gives each friend a
  page: avatar, plain-language status with a next step, last seen, last
  synced, Remove Friend. Removals are staged until OK.
- scripts/cassette_pair.py runs throwaway Syncthings on loopback;
  pytest --syncthing drives a real three-machine handshake.

lintunes/__init__.py

@@ -1,3 +1,3 @@
 """LinTunes — iTunes-style music library manager and player for Linux."""
 
-__version__ = "0.26.0"
+__version__ = "0.27.0"