feat(admin): add admin-configurable default user settings

Allow admins to set instance-wide defaults for temperature unit, color
mode, time format, route calculation, blur booking codes, and map tile
URL via a new Admin > User Defaults tab. Defaults are stored in
app_settings (prefixed default_user_setting_*) and applied at read time
as a fallback — user's own explicit values always take priority.
Translations added for all 16 supported languages.

client/src/pages/AdminPage.tsx

@@ -2,6 +2,7 @@ import React, { useEffect, useState } from 'react'
 import { useNavigate } from 'react-router-dom'
 import apiClient, { adminApi, authApi, notificationsApi } from '../api/client'
 import DevNotificationsPanel from '../components/Admin/DevNotificationsPanel'
+import DefaultUserSettingsTab from '../components/Admin/DefaultUserSettingsTab'
 import { useAuthStore } from '../store/authStore'
 import { useSettingsStore } from '../store/settingsStore'
 import { useAddonStore } from '../store/addonStore'
@@ -169,6 +170,7 @@ export default function AdminPage(): React.ReactElement {
   const TABS = [
     { id: 'users', label: t('admin.tabs.users') },
     { id: 'config', label: t('admin.tabs.config') },
+    { id: 'defaults', label: t('admin.tabs.defaults') },
     { id: 'addons', label: t('admin.tabs.addons') },
     { id: 'settings', label: t('admin.tabs.settings') },
     { id: 'notifications', label: t('admin.tabs.notifications') },
@@ -1493,6 +1495,8 @@ export default function AdminPage(): React.ReactElement {
 
           {activeTab === 'github' && <GitHubPanel isPrerelease={updateInfo?.is_prerelease ?? false} />}
 
+          {activeTab === 'defaults' && <DefaultUserSettingsTab />}
+
           {activeTab === 'dev-notifications' && <DevNotificationsPanel />}
         </div>
       </div>