feat: complete LOW priority code quality improvements

Backend:
- LOW-002: Add Query validation with max page size limits (100)
- LOW-003: Replace magic strings with TaskStatus.is_done flag
- LOW-004: Add 'creation' trigger type validation
- Add action_executor.py with UpdateFieldAction and AutoAssignAction

Frontend:
- LOW-005: Replace TypeScript 'any' with 'unknown' + type guards
- LOW-006: Add ConfirmModal component with A11Y support
- LOW-007: Add ToastContext for user feedback notifications
- LOW-009: Add Skeleton components (17 loading states replaced)
- LOW-010: Setup Vitest with 21 tests for ConfirmModal and Skeleton

Components updated:
- App.tsx, ProtectedRoute.tsx, Spaces.tsx, Projects.tsx, Tasks.tsx
- ProjectSettings.tsx, AuditPage.tsx, WorkloadPage.tsx, ProjectHealthPage.tsx
- Comments.tsx, AttachmentList.tsx, TriggerList.tsx, TaskDetailModal.tsx
- NotificationBell.tsx, BlockerDialog.tsx, CalendarView.tsx, WorkloadUserDetail.tsx

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/App.tsx

@@ -1,5 +1,6 @@
 import { Routes, Route, Navigate } from 'react-router-dom'
 import { useAuth } from './contexts/AuthContext'
+import { Skeleton } from './components/Skeleton'
 import Login from './pages/Login'
 import Dashboard from './pages/Dashboard'
 import Spaces from './pages/Spaces'
@@ -16,7 +17,12 @@ function App() {
   const { isAuthenticated, loading } = useAuth()
 
   if (loading) {
-    return <div className="container">Loading...</div>
+    return (
+      <div className="container" style={{ padding: '24px', maxWidth: '1200px', margin: '0 auto' }}>
+        <Skeleton variant="rect" width="100%" height={60} style={{ marginBottom: '24px' }} />
+        <Skeleton variant="rect" width="100%" height={400} />
+      </div>
+    )
   }
 
   return (