feat: add i18n internationalization support

- Add react-i18next, i18next with browser language detection
- Support Traditional Chinese (zh-TW) and English (en)
- Default language: zh-TW, stored in localStorage
- Create 10 translation namespaces (common, auth, dashboard, tasks, etc.)
- Add LanguageSwitcher component in header
- Translate pages: Login, Dashboard, Tasks, Spaces, Workload, Audit

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

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

frontend/src/pages/Dashboard.tsx

@@ -1,4 +1,5 @@
 import { useEffect, useState, useCallback } from 'react'
+import { useTranslation } from 'react-i18next'
 import { useAuth } from '../contexts/AuthContext'
 import { dashboardApi, DashboardResponse } from '../services/dashboard'
 import {
@@ -10,6 +11,7 @@ import {
 import { Skeleton } from '../components/Skeleton'
 
 export default function Dashboard() {
+  const { t } = useTranslation('dashboard')
   const { user } = useAuth()
   const [data, setData] = useState<DashboardResponse | null>(null)
   const [loading, setLoading] = useState(true)
@@ -22,7 +24,7 @@ export default function Dashboard() {
       const response = await dashboardApi.getDashboard()
       setData(response)
     } catch (err) {
-      setError('Failed to load dashboard data. Please try again.')
+      setError(t('common:messages.networkError'))
       console.error('Dashboard fetch error:', err)
     } finally {
       setLoading(false)
@@ -93,19 +95,19 @@ export default function Dashboard() {
     return (
       <div style={styles.container}>
         <div style={styles.welcomeSection}>
-          <h1 style={styles.welcomeTitle}>Welcome, {user?.name}!</h1>
+          <h1 style={styles.welcomeTitle}>{t('welcome', { name: user?.name })}</h1>
         </div>
 
         <div style={styles.errorCard}>
           <div style={styles.errorIcon}>!</div>
-          <h3 style={styles.errorTitle}>Unable to Load Dashboard</h3>
+          <h3 style={styles.errorTitle}>{t('common:messages.error')}</h3>
           <p style={styles.errorMessage}>{error}</p>
           <button
             style={styles.retryButton}
             onClick={fetchDashboard}
             type="button"
           >
-            Try Again
+            {t('common:buttons.refresh')}
           </button>
         </div>
       </div>
@@ -117,9 +119,9 @@ export default function Dashboard() {
     <div style={styles.container}>
       {/* Welcome Section */}
       <div style={styles.welcomeSection}>
-        <h1 style={styles.welcomeTitle}>Welcome, {user?.name}!</h1>
+        <h1 style={styles.welcomeTitle}>{t('welcome', { name: user?.name })}</h1>
         <p style={styles.welcomeSubtitle}>
-          Here is your work overview for today
+          {t('sections.projectOverview')}
         </p>
       </div>
 
@@ -130,27 +132,27 @@ export default function Dashboard() {
             <StatisticsCard
               icon="✓"
               value={data.task_stats.assigned_count}
-              label="My Tasks"
+              label={t('stats.myTasks')}
               color="#2196f3"
             />
             <StatisticsCard
               icon="⏰"
               value={data.task_stats.due_this_week}
-              label="Due This Week"
+              label={t('deadlines.thisWeek')}
               color="#ff9800"
               highlight={data.task_stats.due_this_week > 0}
             />
             <StatisticsCard
               icon="⚠"
               value={data.task_stats.overdue_count}
-              label="Overdue"
+              label={t('deadlines.overdue')}
               color="#f44336"
               highlight={data.task_stats.overdue_count > 0}
             />
             <StatisticsCard
               icon="✅"
               value={data.task_stats.completion_rate}
-              label="Completion Rate"
+              label={t('stats.completedTasks')}
               color="#4caf50"
               suffix="%"
             />