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/api/client.ts

@@ -299,6 +299,8 @@ export const adminApi = {
     apiClient.post('/admin/dev/test-notification', data).then(r => r.data),
   getNotificationPreferences: () => apiClient.get('/admin/notification-preferences').then(r => r.data),
   updateNotificationPreferences: (prefs: Record<string, Record<string, boolean>>) => apiClient.put('/admin/notification-preferences', prefs).then(r => r.data),
+  getDefaultUserSettings: () => apiClient.get('/admin/default-user-settings').then(r => r.data),
+  updateDefaultUserSettings: (settings: Record<string, unknown>) => apiClient.put('/admin/default-user-settings', settings).then(r => r.data),
 }
 
 export const addonsApi = {