fix(nowplaying): cover leaves with the track, not lingering/expanding on pause

[gyng]

What

Fixes the now-playing widget where, on pause, the album cover expands to fill the box for a moment before disappearing (reported as "the track text disappears, then the cover").

Root cause

The title/artist are derived straight from props, so they vanish the instant a session loses its media. The cover, however, lives in React state and was held by the crossfade's no-art grace (NO_ART_GRACE_MS = 1200) whenever the art URL was absent — including when there was no current track at all (a player that tears down / recreates its SMTC session on pause, so the prior track can't be carried forward). With the title/artist rows gone, the flex: 1 cover stack grew to fill the freed space and lingered up to ~1.2s before fading.

Fix

Tie the cover's lifecycle to whether there's a current track:

• Effect — drop the layers immediately when there is no media (!hasSession || !hasMediaRef.current), not only when there's no session. The no-art grace below now only ever bridges a genuine art-lag (a track is present, its cover just hasn't arrived).
• Render — gate the cover stack on hasMedia, so the cover leaves in the same commit as the title (no transient expand frame).

The idle label (text with no session) and track-to-track crossfade are unchanged.

Test

Adds a failing-first test in NowPlaying.test.tsx using a present session with last_media_update: null. It reproduced the bug (cover held: expected 1 to be 0) and is green after the fix. Existing grace/crossfade tests still pass.

Verification

• npm run check ✓ · npm run lint ✓ (Prettier + ESLint, 0 warnings) · full unit suite 269 files ✓
• Pure frontend change — no Rust/bridge touched.
• Note: a live repro depends on the player actually dropping its SMTC session on pause (browser media tabs do; Spotify/foobar2000 usually keep it and just dim). The unit test pins the path deterministically regardless.

Heads-up for review: NowPlaying.tsx renders as a binary diff on GitHub because the file contains two pre-existing \0 delimiters in trackKey (${source}\0${title}\0${artist}) — unrelated to this change.

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