Add unified All accounts inbox view

[bberenberg]

Summary

• split the unified inbox work out of Adds codex support, Adds multi account view, small perf and security fixes #92 into a focused PR
• add an All accounts inbox mode with persisted account selection
• make inbox actions, threading, search, and sync state account-aware in the unified view

Changes

• add persisted selected account handling, including null for the unified inbox view
• update renderer threading and search flows to operate across accounts while preserving per-account sender identity
• make batch actions, snooze, and undo grouping account-aware for multi-account selections
• update sync buffering and store updates to handle sparse account-aware mutations more efficiently

Tests

• npx tsc --noEmit
• npx playwright test tests/unit/store-selectors.spec.ts tests/unit/optimistic-reads.spec.ts tests/unit/sync-buffer-suppression.spec.ts tests/unit/email-sync.spec.ts

Split from #92.