Post-launch P1: add local save/load for project snapshots

[honor2030]

Goal

Let users pause and resume work without introducing a server-side project database.

Acceptance criteria

• Users can save and reload a project locally (IndexedDB).
• The app can export and import a snapshot file.
• Incompatible snapshot versions fail gracefully.

Task list

• Implement snapshot serializer/deserializer.
• Add save/load UI (button in header).
• Handle versioning errors with a clear message.
• Write tests for snapshot round-trip.

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Estimate: 0.5 day