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-tauri/crates/psysonic-audio/src/decode.rs

@@ -647,6 +647,7 @@ pub(crate) fn build_streaming_source(
     fade_in_dur: Duration,
     sample_counter: Arc<AtomicU64>,
     target_rate: u32,
+    count_gate: Option<Arc<AtomicBool>>,
 ) -> Result<BuiltSource, String> {
     let sample_rate = decoder.sample_rate();
     let channels = decoder.channels();
@@ -685,7 +686,10 @@ pub(crate) fn build_streaming_source(
     let fade_in = EqualPowerFadeIn::new(eq_src, fade_in_dur);
     let fade_out = TriggeredFadeOut::new(fade_in, fadeout_trigger.clone(), fadeout_samples.clone());
     let notifying = NotifyingSource::new(fade_out, done_flag);
-    let counting = CountingSource::new(notifying, sample_counter);
+    let counting = match count_gate {
+        Some(gate) => CountingSource::new_gated(notifying, sample_counter, gate),
+        None => CountingSource::new(notifying, sample_counter),
+    };
     let boosted = PriorityBoostSource::new(counting);
 
     Ok(BuiltSource {
@@ -980,6 +984,7 @@ mod build_source_tests {
             Duration::ZERO,
             sample_counter,
             0,
+            None,
         )
         .expect("build_streaming_source must succeed for a valid WAV decoder");
         assert_eq!(built.output_channels, 1);