feat(login): add language dropdown, browser auto-detection and configurable default

Replace the language cycling button on the login page with a dropdown
showing all 14 supported languages. Add automatic browser/OS language
detection via navigator.languages, falling back to a configurable
DEFAULT_LANGUAGE env var, then 'en' as last resort.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

server/src/app.ts

@@ -6,7 +6,7 @@ import path from 'node:path';
 import fs from 'node:fs';
 
 import jwt from 'jsonwebtoken';
-import { JWT_SECRET } from './config';
+import { JWT_SECRET, DEFAULT_LANGUAGE } from './config';
 import { logDebug, logWarn, logError } from './services/auditLog';
 import { enforceGlobalMfaPolicy } from './middleware/mfaPolicy';
 import { authenticate } from './middleware/auth';
@@ -193,6 +193,7 @@ export function createApp(): express.Application {
   app.use('/api/trips/:tripId/reservations', reservationsRoutes);
   app.use('/api/trips/:tripId/days/:dayId/notes', dayNotesRoutes);
   app.get('/api/health', (_req: Request, res: Response) => res.json({ status: 'ok' }));
+  app.get('/api/config', (_req: Request, res: Response) => res.json({ defaultLanguage: DEFAULT_LANGUAGE }));
   app.use('/api', assignmentsRoutes);
   app.use('/api/tags', tagsRoutes);
   app.use('/api/categories', categoriesRoutes);

server/src/config.ts

@@ -98,3 +98,8 @@ if (_encryptionKey) {
 }
 
 export const ENCRYPTION_KEY = _encryptionKey;
+
+// DEFAULT_LANGUAGE sets the language shown on the login page before the user
+// selects one. Only applies when the user has no saved language preference.
+// Supported values: de, en, es, fr, hu, nl, br, cs, pl, ru, zh, zh-TW, it, ar
+export const DEFAULT_LANGUAGE = process.env.DEFAULT_LANGUAGE || 'en';