Viewer smoothness batch + small fixes

[ljchang]

Summary

Six tasks from the review backlog, each individually reviewed, plus a final whole-branch review whose one integration finding is fixed in the last commit.

Viewer smoothness

• Smooth playback overlay: frame advance now driven by requestVideoFrameCallback while playing (was throttled timeupdate at ~4 Hz) — measured 19 distinct overlay frames per 2 s of playback vs ~8 before
• O(1) frame lookups: fex rows and identity assignments indexed by frame once per session (was an O(N) filter over ~1,500-key rows on every frame tick); drag-seeks in the scrub bar and timeseries plot coalesced to animation frames
• Race-free session switching: token-guarded selectSession — rapid A→B clicks can no longer leave A's data under B's id; the big CSV/frame-times fetches start in parallel with the metadata round-trips; loading indicator; failed loads are caught; stale overlay data cleared eagerly so old-session landmarks never render over the new video
• Letterbox-correct face clicks: hit-testing now maps through the aspect-locked box the video/overlay actually fill (was the outer stage — wrong scale AND offset when letterboxed)
• User seeks pause playback (scrub drag, plot click, annotation click) — resolves the final review's finding that the new 60 Hz frame advance and 60 Hz coalesced drag-seeks fought over the playhead

Features & fixes

• "Open in Viewer" works: Analyze's button (previously silently dropped) and a new "Recording saved — Open in Viewer" toast in Live both thread the session id into the Viewer's initial selection
• MeshCanvas ⏸/▶ emoji buttons → lucide icons (repo icon rule)
• Live frame cache 40 → 16 bitmaps (~150 MB → ~60 MB worst case, still 2× the documented latency headroom)
• Recorder no longer KeyErrors writing metadata for a bare RecorderConfig (capabilities omitted instead)
• api.ts health type gains the app identity field; splash "not responding after 30s" hint reworded for first-run cold imports; session-id extraction is path-separator-safe

Test plan

• Full pytest suite: 220 passed (1 new TDD test for the recorder guard)
• pnpm check (only the pre-existing vendor/ogl.js declaration error) + pnpm build
• Browser-driven checks per task against the live sidecar: playback frame-rate measurement, keyboard/scrub sanity, rapid session-switch race test, center-click assign dialog, mid-playback seek pauses, tab cycling with zero console errors