Add local autosave and session recovery

[laubonghaudoi]

Summary

Add local autosave and recovery for subtitle editing sessions. This strengthens the app's offline/PWA promise without adding accounts, cloud storage, or backend state.

Why this is in scope

The app is intentionally local-first and browser-based. Losing unsaved subtitle edits on refresh or crash is a major workflow risk, and a local recovery layer directly supports the existing privacy/design principles.

Initial scope

Persist subtitle project state locally:

• Tracks and track names.
• Active track ID.
• Subtitle text/timing data.
• VTT header/prologue metadata.
• Relevant editor preferences like track labels and QA thresholds if available.

Do not persist media file contents in the first version.

Implementation plan

1. Define a project snapshot schema.

   • Create lib/local-session.ts.
   • Include a schema version, timestamp, app version if available, and track state.
   • Keep media file names as display-only metadata if useful, but do not assume the file can be reopened.

2. Choose storage.

   • Use localStorage for a first minimal implementation if snapshots are small enough.
   • Consider IndexedDB if track payloads become large.
   • Namespaced key: subtitle-editor:autosave:v1.

3. Debounced persistence.

   • Save after subtitle/track changes with a debounce, e.g. 500-1000ms.
   • Avoid writing on every playback tick.
   • Persist only serializable project state, not undo stacks in the first version unless straightforward.

4. Recovery UX.

   • On app load, detect an autosaved session newer than the empty/default state.
   • Show a small recovery banner/dialog: restore, discard, or download backup.
   • After restore, keep normal editing and saving flows.

5. Privacy and safety.

   • Add clear copy that autosave is local to the browser only.
   • Provide clear autosave after successful manual download or from settings.
   • Handle corrupt/old snapshots gracefully.

6. Tests.

   • Add unit tests for serialization/deserialization and version handling.
   • Add component tests for restore/discard if feasible.

Acceptance criteria

• Edits are saved locally after changes without a backend.
• Reloading the page offers to restore the previous subtitle session.
• Users can discard or clear the local autosave.
• Media files are not silently uploaded or persisted.