🔒 Disable self-registration and compact admin interface

Security improvements:
- Removed registration link from login page
- Disabled /auth/register route - redirects with error message
- Removed demo credentials from login page
- Added info message: 'New users are created by administrators'

UI improvements:
- Compacted all admin interface buttons (btn-sm)
- Reduced heading sizes (H2 → H4) for less visual dominance
- Shortened badge texts ('Administrator' → 'Admin', 'Benutzer' → 'User')
- Optimized spacing and reduced margins/paddings
- Cleaner, more professional admin interface

Access control:
- Only administrators can create new users via admin panel
- Self-registration completely disabled for security
- Maintains full admin functionality with improved UX

app/templates/auth/login.html

@@ -30,17 +30,13 @@
                     </div>
                 </form>
             </div>
-            <div class="card-footer text-center">
-                <a href="{{ url_for('auth.register') }}" class="text-decoration-none">
-                    Noch kein Konto? Hier registrieren
-                </a>
-            </div>
         </div>
         
-        <div class="mt-3 p-3 bg-light rounded">
-            <h6>Demo-Zugang:</h6>
-            <p class="small mb-1"><strong>Benutzername:</strong> admin</p>
-            <p class="small mb-0"><strong>Passwort:</strong> admin123</p>
+        <div class="mt-3 p-2 bg-info text-white rounded">
+            <p class="small mb-0">
+                <i class="fas fa-info-circle"></i> 
+                Neue Benutzer werden von Administratoren erstellt.
+            </p>
         </div>
     </div>
 </div>