Fix stale thread route recovery

[redxzeta]

Closes #127

Summary

• gate thread-detail leases to hydrated server threads or registered local drafts
• atomically clear matching remembered routes after authoritative missing-thread recovery and notify same-window consumers
• warm route code with TanStack Router chunk-only loading, avoiding route loaders and side effects
• cover valid, stale, remembered, deleted, and draft-safe recovery paths

Verification

• npx --yes bun@1.3.12 run --cwd apps/web test -- Sidebar.uiState.test.ts threadDetailSubscriptionRetention.test.ts chatRouteRestore.test.ts chatRouteRecovery.test.ts (33 tests passed)
• git diff --check
• targeted local browser startup reached the environment memory ceiling before execution; exact-head GitHub browser checks are required before merge

No deployment or runtime activation is included.