feat(team-inbox): complete collaborative session handoff

[beruro]

Summary

• adds a unified Team Inbox for assignments and mentions
• shares one collaborative Work Item thread surface across Inbox and formal detail views
• completes Session → review → Work Item → teammate handoff → accept/return reconciliation
• preserves singleton ChatPanel navigation, unread state, retries, persisted handoff state, and collaboration projection
• cleanly extracts the Team Inbox scope from perf(session): batch persisted turn-intent status reads #512; no RPC, Onboarding, performance-refactor, or Kanban-refresh source changes are included

Verification

• pnpm run typecheck
• changed TypeScript/TSX ESLint (--max-warnings=0)
• 51 focused Vitest files / 305 tests
• 14 Team Inbox Rust tests
• 7 handoff Rust tests
• cargo check --manifest-path src-tauri/Cargo.toml -p org2
• changed Rust files pass rustfmt --check
• git diff --check

Audit

Architecture, frontend UI, and performance lifecycle reports are included under docs/*audit* and docs/org2-performance-guard-*. The clean-develop extraction report is docs/architecture-audit-2026-07-29/TeamInboxDevelopIntegration.md.

Remaining operational QA

Rendered two-account QA should still verify real-device collaboration timing, account switching, and human-readable errors. The code and persistence lifecycle are covered by focused regression tests.