feat: add getBaseUrl function for consistent base URL generation across routes

feat(calendar): display local timezone in calendar view
feat(i18n): add timezone label to German and English translations

server/routes/oauth.js

@@ -19,7 +19,7 @@ import { Router } from 'express';
 import { rateLimit } from 'express-rate-limit';
 import { v4 as uuidv4 } from 'uuid';
 import { getDb } from '../config/database.js';
-import { generateToken } from '../middleware/auth.js';
+import { generateToken, getBaseUrl } from '../middleware/auth.js';
 import { log } from '../config/logger.js';
 import {
   getOAuthConfig,
@@ -91,7 +91,7 @@ router.get('/authorize', async (req, res) => {
     const state = await createOAuthState('oidc', codeVerifier, returnTo);
 
     // Build callback URL
-    const baseUrl = process.env.APP_URL || `${req.protocol}://${req.get('host')}`;
+    const baseUrl = getBaseUrl(req);
     const redirectUri = `${baseUrl}/api/oauth/callback`;
 
     // Build authorization URL
@@ -119,7 +119,7 @@ router.get('/callback', callbackLimiter, async (req, res) => {
     const { code, state, error: oauthError, error_description } = req.query;
 
     // Build frontend error redirect helper
-    const baseUrl = process.env.APP_URL || `${req.protocol}://${req.get('host')}`;
+    const baseUrl = getBaseUrl(req);
     const errorRedirect = (msg) =>
       res.redirect(`${baseUrl}/oauth/callback?error=${encodeURIComponent(msg)}`);
 
@@ -253,7 +253,7 @@ router.get('/callback', callbackLimiter, async (req, res) => {
     res.redirect(`${baseUrl}/oauth/callback?token=${encodeURIComponent(token)}&return_to=${encodeURIComponent(returnTo)}`);
   } catch (err) {
     log.auth.error(`OAuth callback error: ${err.message}`);
-    const baseUrl = process.env.APP_URL || `${req.protocol}://${req.get('host')}`;
+    const baseUrl = getBaseUrl(req);
     res.redirect(`${baseUrl}/oauth/callback?error=${encodeURIComponent('OAuth authentication failed. Please try again.')}`);
   }
 });