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/components/KanbanBoard.tsx

@@ -288,7 +288,7 @@ const styles: Record<string, React.CSSProperties> = {
     color: '#666',
   },
   subtaskBadge: {
-    color: '#999',
+    color: '#767676', // WCAG AA compliant
   },
   customValueBadge: {
     backgroundColor: '#f3e5f5',
@@ -304,7 +304,7 @@ const styles: Record<string, React.CSSProperties> = {
   emptyColumn: {
     textAlign: 'center',
     padding: '24px',
-    color: '#999',
+    color: '#767676', // WCAG AA compliant
     fontSize: '13px',
     border: '2px dashed #ddd',
     borderRadius: '6px',