fix: change trial banner button to mailto contact link

Replace subscription selection link with email contact for now.
Future: Central subscription system on jinkendo.de for all apps.

Button text:
- "Abo wählen" → "Abo anfragen"
- "Jetzt upgraden" → "Kontakt aufnehmen"

Opens mailto:mitai@jinkendo.de with pre-filled subject and body.

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

frontend/src/components/TrialBanner.jsx

@@ -64,8 +64,8 @@ export default function TrialBanner({ profile }) {
         </div>
       </div>
 
-      <Link
-        to="/settings?tab=subscription"
+      <a
+        href="mailto:mitai@jinkendo.de?subject=Abo-Anfrage%20für%20Mitai%20Jinkendo&body=Hallo,%0A%0Aich%20möchte%20gerne%20ein%20Abo%20für%20Mitai%20Jinkendo%20abschließen.%0A%0AMein%20Profil:%20"
         style={{
           padding: '10px 20px',
           borderRadius: 8,
@@ -76,11 +76,12 @@ export default function TrialBanner({ profile }) {
           textDecoration: 'none',
           whiteSpace: 'nowrap',
           border: 'none',
-          cursor: 'pointer'
+          cursor: 'pointer',
+          display: 'inline-block'
         }}
       >
-        {isUrgent ? 'Jetzt upgraden' : 'Abo wählen'}
-      </Link>
+        {isUrgent ? 'Kontakt aufnehmen' : 'Abo anfragen'}
+      </a>
     </div>
   )
 }