refactor: split monolithic api.ts into domain modules, extract AppShell calendar/mobile concerns
- Split frontend/src/lib/api.ts (2042 lines) into 12 focused domain modules:
client.ts (base HTTP/auth), calendar.ts, documents.ts, employees.ts,
equipment.ts, feed.ts, messages.ts, notifications.ts, procurement.ts,
requests.ts, utils.ts, and index.ts barrel that preserves all existing imports
- Extract calendar modal state from AppShell into useCalendarModals hook
- Extract CalendarModals and MobileDrawers into components/layout/
- AppShell: 745 → 460 lines, focused on layout concerns only
- Fix setState-in-effect lint error with startTransition pattern
- Stabilize notifications memo to prevent unnecessary re-renders
- Fix flaky WS test: handle CancelledError in channels-testing cleanup
- All 609 backend tests pass, TypeScript typecheck clean, ESLint clean
3ca4d108b6