`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>
