fix(seekbar): commit seek on mouseup + ignore progress ticks while dragging

Ports the WaveformSeek UX changes from PR #219 (original fix by @cucadmuh)
onto test/seekable-streaming:

- Split previewFraction (during drag) / commitSeek (on mouseup) so audio_seek
  fires once per gesture instead of on every mousemove.
- Subscribe-effect skips external progress updates while isDragging — prevents
  cursor flicker from streaming/recovery ticks fighting the dragged position.
- Clear seekTarget in seek() catch branch so a failed seek doesn't permanently
  block progress updates.

Skips the byte-restart fast-path from #219: would defeat RangedHttpSource's
direct seek by restarting the stream after 450ms.

Co-Authored-By: cucadmuh <cucadmuh@users.noreply.github.com>
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/store/playerStore.ts

@@ -1427,6 +1427,9 @@ export const usePlayerStore = create<PlayerState>()(
           seekDebounce = null;
           seekTarget = time;
           invoke('audio_seek', { seconds: time }).catch((err: unknown) => {
+            // Release the progress-tick guard so the UI doesn't freeze
+            // waiting for a target the engine will never reach.
+            seekTarget = null;
             const msg = String(err ?? '');
             if (!msg.includes('not seekable')) {
               console.error(err);