feat: add multi-project management

[britle20]

Summary

• Add browser-local multi-project persistence with project collection metadata and active project restore.
• Add a transport bar project menu for creating, selecting, renaming, and deleting local projects.
• Scope imported WAV blobs by project ID so separate projects can reuse local sample IDs without collisions.
• Update docs and persistence helper tests for the multi-project model.

Changes

• Added project summaries, active project ID state, project create/select/rename/delete handlers, and autosave by active project ID.
• Added IndexedDB project metadata and project-scoped imported sample blob storage with legacy single-project migration.
• Updated the sidebar and transport UI to display the active project name from state.
• Updated data model and architecture docs for scoped project persistence.

Testing

• npm run typecheck
• npm run lint
• npm run test
• npm run build

Screenshots

Not captured. Manual server smoke check: npm run dev -- --host 127.0.0.1 --port 5173 started successfully and curl -I http://127.0.0.1:5173/ returned HTTP 200.

Notes / Limitations

• Project create/rename/delete currently uses browser prompt/confirm dialogs rather than custom modals.
• Project management is browser-local IndexedDB only; no cloud sync or file-system project browser.
• Legacy single-project data is migrated to the default local project where practical.

Follow-up

• Consider custom project management dialogs if prompt/confirm becomes too limiting.

Closes #59