feat(playback): stream buffering UI, M4A moov-at-end streaming, hot-cache spill (#737)

* feat(playback): stream buffering UI, ranged M4A tail prefetch, demuxer fix

Defer seekbar/progress until HTTP stream is armed for both legacy and
RangedHttpSource; show buffering overlay on cover art. Add MP4 tail
prefetch and Symphonia isomp4 bounded-mdat/moov-at-EOF probing so
moov-at-end M4A can start without reading the full mdat.

* feat(hot-cache): spill large ranged streams to disk for promote

When a ranged HTTP download completes above the 64 MiB RAM promote cap,
write the existing buffer once to app-data stream-spill/ and register it
for hot-cache promote (rename) and replay via fetch_data. Analysis seeds
from the spill file up to the local-file cap (512 MiB).

* fix(ui): stream buffering — grayscale cover and static clock icon

Desaturate player and queue cover art while isPlaybackBuffering; keep a
non-animated clock overlay for visibility without the spinning animation.

* fix(playback): review follow-up — tests, i18n, spill cleanup, changelog

Clippy and test layout fixes; stream spill orphan cleanup on startup;
buffering flag guard in progress handler; bufferingStream in all player
locales; CHANGELOG and contributor credits for stream/M4A work.

* docs: attribute stream buffering and M4A streaming to PR #737

* test(audio): avoid create_engine in stream spill unit test

CI runners have no audio output device; test spill take/consume via
the Mutex slot only, matching install_stream_completed_spill tests.

src/store/playTrackAction.ts

@@ -254,7 +254,10 @@ export function runPlayTrack(
       currentTime: initialTime,
       scrobbled: false,
       lastfmLoved: false,
-      isPlaying: true, // optimistic — reverted on error
+      // HTTP stream: wait for Rust `audio:playing` so the seekbar does not
+      // extrapolate while RangedHttpSource / legacy reader is still buffering.
+      isPlaying: playbackSourceHint !== 'stream',
+      isPlaybackBuffering: playbackSourceHint === 'stream',
       currentPlaybackSource: playbackSourceHint,
       enginePreloadedTrackId: keepPreloadHint ? track.id : null,
     });