feat(team-inbox): add unified inbox with lifecycle fixes

[beruro]

Summary

• reintroduce the viewer-scoped Team Inbox after the original merge was reverted
• merge local Work Item assignments with validated managed-cloud comment mentions
• add the master/detail surface, unread/read actions, filters, search, pagination, canonical navigation, and singleton Chat Panel/Sidebar integration
• include the follow-up React lifecycle fixes: pure recency grouping, request-keyed body loading, stale-response rejection, and stable realtime teardown capture

Validation

• focused Vitest suite: 87 tests passed
• tsc --noEmit
• full pnpm run lint
• cargo test --manifest-path src-tauri/Cargo.toml -p project_management team_inbox --no-fail-fast: 12 passed
• cargo check --manifest-path src-tauri/Cargo.toml -p org2
• git diff --check

Audits

• Architecture: all 10 layers covered; viewer IDs, tagged DTOs, command parity, and schema initialization verified
• Frontend UI (feature): 7 fixes, 4 keep with reason, 0 abstraction candidates
• Frontend UI (lifecycle follow-up): 2 fixes, 4 keep with reason, 0 abstraction candidates
• Performance: PASS. No polling; refresh remains demand/signal-driven, concurrent reads are guarded, stale loads are rejected, subscriptions/timers have teardown, and cloud receipts remain capped at 1,000

Supersedes the reverted #521 and obsolete follow-up #529.