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/views/Lyrics.vue

@@ -136,7 +136,9 @@ function scrollToActiveLine(idx: number) {
 
 function syncLyrics() {
   if (!store.isPlaying || lines.value.length === 0) return;
-  const elapsed = store.elapsed;
+  // liveElapsed() (action) is recomputed now; the cached `elapsed` getter only
+  // refreshed on server pushes, leaving highlights ~half a line behind (#107).
+  const elapsed = store.liveElapsed();
   const idx = findActiveLine(elapsed);
   // Only update when the active line actually changes
   if (idx !== activeLine.value && idx >= 0) {