v0.29.0: Cassette friend mode — browse a friend's library, cassette songs

- A ▾ cut from the right end of the Library button (shown once there's a
  friend) opens a popup of friends. Choosing one enters friend mode: the
  button becomes "<Name>'s Library" in light purple, the sidebar lists
  their shared playlists with a UFO follow toggle on each, and the content
  area is their library — browse only, nothing plays or edits.
- A cassette column on the far left and cassette buttons on every artist
  and album: pressed grey = send me this, medium grey on hover to let go.
  An artist cassettes all its albums and tracks (a snapshot).
- Bottom bar: Hide tracks I have (exact matches: artist, album, title,
  length within 2 s); close matches show light grey. Size of what's
  cassetted against free space: orange when Syncthing would stall at its
  free-space floor, red when it won't fit.
- Save and Close writes requests.json; Cancel discards; the ▾ or quitting
  with changes asks Save / Don't Save / Cancel.

lintunes/__init__.py

@@ -1,3 +1,3 @@
 """LinTunes — iTunes-style music library manager and player for Linux."""
 
-__version__ = "0.28.0"
+__version__ = "0.29.0"