v0.4.0: cast to a Chromecast from the Connections menu

The Device menu becomes Connections, with "Connect to Chromecast…" beside
the Rabbit sync. It opens a dialog that spins while it searches and lists
devices as they appear; picking one hands playback over, puts a small cast
glyph under the volume slider, and clicking that glyph disconnects.

Media-receiver model: lintunes serves the original file over the LAN on an
ephemeral port and the Chromecast decodes it itself. Bit-exact — no
transcode, no second lossy encode — and the device buffers for itself. The
cost is that lintunes is a remote control while connected: no local PCM, so
the visualizer shows the cast glyph instead of bars, and transport actions
land with about a second of round trip.

Player now walks its queue through a swappable PlaybackSink. It keeps
owning the queue, shuffle walk, start/stop times and the play-count and
scrobble bookkeeping, so casting counts plays and scrobbles exactly like
local playback. set_sink() carries track, position and playing-state both
ways, so connecting and disconnecting pick up mid-song. LocalSink stays in
player.py because the Player tests stub Qt Multimedia in that namespace.

The URL carries an opaque random token, never a path, so there is nothing
to traverse with; only the last few played tracks stay resolvable and the
whole map dies with the session. Range and HEAD are implemented because
the device seeks by re-requesting ranges and won't report a duration
without them.

Failure handling, verified against a real device: a dropped socket gets a
15s grace period, since pychromecast retries on its own and a Wi-Fi blip
heals itself. A real loss, another app taking the device, or a network
change falls back to local playback still playing, at the same position —
the sink reports the state lintunes last asked for rather than the IDLE
status a dying connection pushes just before it goes. Quitting stops the
device instead of leaving it fetching from a server that just died. The
~119 Apple Lossless / AIFF / protected-AAC tracks are skipped with a
status-bar message; the other 21,000+ MP3 and AAC files cast natively.

New dependency: pychromecast>=14.0.10, imported lazily so the app still
launches where it isn't installed (the menu item then explains the
install), and python_requires raised to >=3.11 to match its floor.

NOTE: the other machine needs `pip install -e .` before casting appears.

tests/test_round29.py: 67 tests; 440 pass overall.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tests/test_player.py

@@ -56,7 +56,7 @@ def test_missing_file_emits_track_missing_and_stops(qapp, tmp_path):
     assert missing == [track]
     # Stopped, not advanced into a track; setSource was never called.
     assert player.current_track is None
-    player._media.setSource.assert_not_called()
+    player._local._media.setSource.assert_not_called()
 
 
 def test_all_missing_queue_does_not_recurse(qapp, tmp_path):
@@ -104,4 +104,4 @@ def test_retry_current_plays_after_relocate(qapp, tmp_path):
 
     assert errors == [] and missing == []
     assert player.current_track is track
-    player._media.setSource.assert_called_once()
+    player._local._media.setSource.assert_called_once()