Replace dormant Bluesky model with isolated local accounts

[micahflee]

Closes #673

Summary

• replace the dormant Bluesky schema with UUID-keyed local-account metadata and immutable unique DIDs
• add private per-account runtime databases, durable job state, content-addressed media, and staging roots
• initialize local storage on creation/reopen and require UUID-confirmed cleanup of the account partition and resources
• add forward-migration, controller lifecycle/isolation, IPC cleanup, and renderer transition coverage

Verification

• npm run lint
• npx vitest run src --silent=true

The repository test wrapper could not launch in this Linux execution environment because the installed Electron artifact is the macOS binary; the complete src test set passed through direct Vitest.

(This was written by an LLM.)