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.

server/src/routes/admin.ts

@@ -3,6 +3,7 @@ import { authenticate, adminOnly } from '../middleware/auth';
 import { AuthRequest } from '../types';
 import { writeAudit, getClientIp, logInfo } from '../services/auditLog';
 import * as svc from '../services/adminService';
+import { getAdminUserDefaults, setAdminUserDefaults } from '../services/settingsService';
 import { invalidateMcpSessions } from '../mcp';
 import { getPreferencesMatrix, setAdminPreferences } from '../services/notificationPreferencesService';
 
@@ -346,6 +347,31 @@ router.post('/rotate-jwt-secret', (req: Request, res: Response) => {
   res.json({ success: true });
 });
 
+// ── Default User Settings ──────────────────────────────────────────────────────
+
+router.get('/default-user-settings', (_req: Request, res: Response) => {
+  res.json(getAdminUserDefaults());
+});
+
+router.put('/default-user-settings', (req: Request, res: Response) => {
+  if (!req.body || typeof req.body !== 'object' || Array.isArray(req.body)) {
+    return res.status(400).json({ error: 'Object body required' });
+  }
+  try {
+    setAdminUserDefaults(req.body);
+    const authReq = req as AuthRequest;
+    writeAudit({
+      userId: authReq.user.id,
+      action: 'admin.default_user_settings_update',
+      ip: getClientIp(req),
+      details: req.body,
+    });
+    res.json(getAdminUserDefaults());
+  } catch (err: any) {
+    res.status(400).json({ error: err.message });
+  }
+});
+
 // ── Dev-only: test notification endpoints ──────────────────────────────────────
 if (process.env.NODE_ENV === 'development') {
   const { send } = require('../services/notificationService');