[cbaucom]

Closes #11

Approach

Folder audio (MP3/M4A/WAV) is detected separately from visual media and offered as soundtrack choices in a new SoundtrackPanel. Selection is persisted as soundtrackFilename in slideshow.json. The Sequence Planner attaches a constant-volume soundtrack to RenderPlan; the Composition plays it from frame 0 via Html5Audio (Player-safe with blob URLs).

Pure modules (TDD)

• timeline-core/media.ts — isSupportedAudio(); visual isSupportedMedia() unchanged (audio excluded from storyboard)
• sequence-planner/planner.ts — optional SoundtrackInput → RenderPlan.soundtrack with volume: 1; totalFrames independent of track length
• project-store/audio-loader.ts — enumerateAudioTracks() with Mediabunny duration; blob URL lifecycle helpers
• project-store/schema.ts — soundtrackFilename?: string on SlideshowJson

UI

• SoundtrackPanel — dropdown when folder contains audio; hidden when none
• App.tsx — parallel audio enumeration on folder open/refresh; autosave includes soundtrack; plan wired with selected track

Playback fix

• @remotion/media Audio blocks the Player on WebCodecs decode for blob URLs (delayRender never completes). Switched soundtrack to Html5Audio from remotion inside AbsoluteFill.

Acceptance criteria

• Given audio files in the folder, when the project opens, then they are listed as soundtrack options distinct from visual media (and excluded from the storyboard grid). (Separate enumerateAudioTracks; SoundtrackPanel lists tracks; storyboard shows photos only — Playwright smoke)
• Given a selected soundtrack, when played, then music starts with the show and the selection persists across reopen. (Html5Audio at frame 0; soundtrackFilename in slideshow.json; validated on reopen against enumerated tracks)
• Given a show longer than the track, when playback passes the track end, then audio ends gracefully (no crash, no loop). (No loop prop; totalFrames unchanged by track duration; planner test confirms independence)

Code review verdict

APPROVE (4/5)

Blocking findings: none

Non-blocking: App.tsx now ~358 lines (over 300-line target); audio-loader.ts has no colocated FS-fake tests (same gap as media-loader.ts); SoundtrackTrack.durationInFrames stored but not consumed by Composition yet (scaffolding for beat sync).

Test plan

• pnpm test (135 tests)
• pnpm build
• pnpm lint
• Playwright smoke: open test-fixtures/demo, select theme.wav, play advances (pause button appears)

Follow-ups

• Extract audio blob lifecycle from App.tsx into a hook when next touching editor shell
• Consider empty-state copy in SoundtrackPanel when folder has no audio (panel currently hidden)

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