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/WorkloadUserDetail.tsx

@@ -1,5 +1,6 @@
-import { useState, useEffect } from 'react'
+import { useState, useEffect, useRef, useCallback } from 'react'
 import { UserWorkloadDetail, LoadLevel, workloadApi } from '../services/workload'
+import { SkeletonList } from './Skeleton'
 
 interface WorkloadUserDetailProps {
   userId: string
@@ -34,14 +35,27 @@ export function WorkloadUserDetail({
   const [loading, setLoading] = useState(true)
   const [error, setError] = useState<string | null>(null)
   const [detail, setDetail] = useState<UserWorkloadDetail | null>(null)
+  const modalOverlayRef = useRef<HTMLDivElement>(null)
 
+  // A11Y: Handle Escape key to close modal
   useEffect(() => {
-    if (isOpen && userId) {
-      loadUserDetail()
+    const handleKeyDown = (e: KeyboardEvent) => {
+      if (e.key === 'Escape' && isOpen) {
+        onClose()
+      }
     }
-  }, [isOpen, userId, weekStart])
 
-  const loadUserDetail = async () => {
+    if (isOpen) {
+      document.addEventListener('keydown', handleKeyDown)
+      modalOverlayRef.current?.focus()
+    }
+
+    return () => {
+      document.removeEventListener('keydown', handleKeyDown)
+    }
+  }, [isOpen, onClose])
+
+  const loadUserDetail = useCallback(async () => {
     setLoading(true)
     setError(null)
     try {
@@ -53,7 +67,13 @@ export function WorkloadUserDetail({
     } finally {
       setLoading(false)
     }
-  }
+  }, [userId, weekStart])
+
+  useEffect(() => {
+    if (isOpen && userId) {
+      loadUserDetail()
+    }
+  }, [isOpen, userId, weekStart, loadUserDetail])
 
   const formatDate = (dateStr: string | null) => {
     if (!dateStr) return '-'
@@ -64,11 +84,19 @@ export function WorkloadUserDetail({
   if (!isOpen) return null
 
   return (
-    <div style={styles.overlay} onClick={onClose}>
+    <div
+      ref={modalOverlayRef}
+      style={styles.overlay}
+      onClick={onClose}
+      tabIndex={-1}
+      role="dialog"
+      aria-modal="true"
+      aria-labelledby="workload-detail-title"
+    >
       <div style={styles.modal} onClick={(e) => e.stopPropagation()}>
         <div style={styles.header}>
           <div>
-            <h2 style={styles.title}>{userName}</h2>
+            <h2 id="workload-detail-title" style={styles.title}>{userName}</h2>
             <span style={styles.subtitle}>Workload Details</span>
           </div>
           <button style={styles.closeButton} onClick={onClose} aria-label="Close">
@@ -77,7 +105,9 @@ export function WorkloadUserDetail({
         </div>
 
         {loading ? (
-          <div style={styles.loading}>Loading...</div>
+          <div style={{ padding: '16px' }}>
+            <SkeletonList count={5} showAvatar={false} />
+          </div>
         ) : error ? (
           <div style={styles.error}>{error}</div>
         ) : detail ? (
@@ -203,7 +233,7 @@ const styles: { [key: string]: React.CSSProperties } = {
     border: 'none',
     fontSize: '28px',
     cursor: 'pointer',
-    color: '#999',
+    color: '#767676', // WCAG AA compliant
     padding: '0',
     lineHeight: 1,
   },
@@ -265,7 +295,7 @@ const styles: { [key: string]: React.CSSProperties } = {
   emptyTasks: {
     textAlign: 'center',
     padding: '24px',
-    color: '#999',
+    color: '#767676', // WCAG AA compliant
     fontSize: '14px',
   },
   taskList: {