Preserve Bluesky media for offline browsing

[micahflee]

Closes #90

Summary

• download original image and video blobs from each author’s PDS into UUID-keyed account storage
• track content identity, local references, source metadata, completion state, failures, and retry attempts in the account database
• deduplicate assets by CID within each account while preserving account isolation
• render and play only local saved media in Browse, with explicit incomplete-media messaging
• retry failed assets through idempotent re-saving

Verification

• npm run lint
• npm test -- --runInBand (45 suites, 634 tests)

(This was written by an LLM.)