fix(web): tick player time every frame and keep lyrics in sync (#107)

`store.elapsed` is a Pinia getter (a cached Vue computed) that interpolates
with `Date.now()`. Because `Date.now()` is not a reactive dependency, the
computed only re-ran on WebSocket pushes / the 3s server poll, so the bottom
progress bar jumped ~3s at a time and lyric highlighting lagged ~half a line —
even though the consumers read it from a 60fps requestAnimationFrame loop.

Add a pure `interpolateElapsed()` helper and a non-cached `liveElapsed()` store
action. The per-frame consumers now call `liveElapsed()` so the value advances
every frame instead of returning a frozen cache:
- web/src/components/Player.vue  (desktop progress bar, rAF)
- web/src/App.vue                (mobile progress bar, rAF)
- web/src/views/Lyrics.vue       (lyric highlight, 500ms interval)

pause() now freezes at the live value rather than a possibly-stale cached one.
The `elapsed` getter is refactored onto the same helper (behaviour unchanged).

Adds web/src/stores/elapsed.test.ts covering the time-advancing interpolation,
paused freeze, no-anchor, and duration-clamp cases.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

web/src/App.vue

@@ -118,8 +118,10 @@ let mobileRaf: number | null = null;
 
 function updateMobileProgress() {
   const duration = currentSong.value?.duration ?? 0;
+  // liveElapsed() recomputes each frame; the cached `elapsed` getter would
+  // leave the mobile bar frozen between server pushes (#107).
   mobileProgressPct.value = duration > 0
-    ? Math.min((playerStore.elapsed / duration) * 100, 100)
+    ? Math.min((playerStore.liveElapsed() / duration) * 100, 100)
     : 0;
   mobileRaf = requestAnimationFrame(updateMobileProgress);
 }