feat: persist active project in IndexedDB

[britle20]

Summary

• Adds an IndexedDB-backed project store with a versioned active project document and a separate imported sample blob store.
• Restores project state on startup and debounces autosave for clips, arrangement placement, tempo, loop range, and mixer state.
• Rebuilds imported audio runtime buffers from stored blobs on demand instead of storing runtime objects in project JSON.
• Adds a minimal transport-bar save status and updates the IndexedDB project persistence #47 feature spec/docs.

Changes

• New persistence helpers in src/persistence/project-store.ts.
• App-level restore/autosave wiring and imported audio blob persistence.
• Audio engine importSampleBlob API for restoring imported samples from IndexedDB blobs.
• Unit coverage for versioned project document helpers and imported sample ID extraction.

Testing

• npm run typecheck --if-present: pass
• npm run lint --if-present: pass
• npm run test --if-present: pass, 13 files / 77 tests
• npm run build --if-present: pass
• npm run dev -- --host 127.0.0.1 --port 5173: started on 5174 because 5173 was occupied; curl -I http://127.0.0.1:5174/ returned 200 OK

Notes / Limitations

• Browser-level manual refresh verification still needs user review because I cannot interact with IndexedDB through the running UI here.
• This stores one active local project only; multi-project management and cloud sync remain out of scope.
• Imported sample bytes are stored as IndexedDB blobs keyed by sampleId; AudioBuffer, File, object URL, and decoded runtime objects remain outside project JSON.

Closes #47