Refactor code and improve internationalization support

- Updated import statements to remove invisible characters.
- Standardized comments to use a consistent hyphen format.
- Adjusted username validation error messages for consistency.
- Enhanced email sending functions to include language support.
- Added email internationalization configuration for dynamic translations.
- Updated calendar and federation routes to include language in user queries.
- Improved user feedback messages in German and English for clarity.

server/routes/admin.js

@@ -1,4 +1,4 @@
-import { Router } from 'express';
+import { Router } from 'express';
 import bcrypt from 'bcryptjs';
 import { v4 as uuidv4 } from 'uuid';
 import { getDb } from '../config/database.js';
@@ -26,7 +26,7 @@ router.post('/users', authenticateToken, requireAdmin, async (req, res) => {
 
     const usernameRegex = /^[a-zA-Z0-9_-]{3,30}$/;
     if (!usernameRegex.test(name)) {
-      return res.status(400).json({ error: 'Username may only contain letters, numbers, _ and - (3–30 chars)' });
+      return res.status(400).json({ error: 'Username may only contain letters, numbers, _ and - (3-30 chars)' });
     }
 
     if (password.length < 8) {
@@ -211,7 +211,7 @@ router.post('/invites', authenticateToken, requireAdmin, async (req, res) => {
 
     if (isMailerConfigured()) {
       try {
-        await sendInviteEmail(email.toLowerCase(), inviteUrl, appName);
+        await sendInviteEmail(email.toLowerCase(), inviteUrl, appName, 'en');
       } catch (mailErr) {
         log.admin.warn(`Invite email failed (non-fatal): ${mailErr.message}`);
       }