fix: Back-Button Fallback auf Dashboard wenn kein History-Eintrag

- router.js: Back-Buttons navigieren zu data-page oder dashboard statt history.back()
- chat-page.js: Chat Back-Button mit gleichem Fallback
- Behebt Problem bei Direktlinks (z.B. #chat/123) ohne vorherigen SPA-Eintrag

wwwroot/js/pages/chat-page.js

@@ -267,7 +267,13 @@ registerPage('chat', {
     // Zurück zum Dashboard
     const backBtn = document.getElementById('chatBackBtn');
     if (backBtn) {
-      backBtn.addEventListener('click', () => history.back());
+      backBtn.addEventListener('click', () => {
+        if (history.state && history.length > 2) {
+          history.back();
+        } else {
+          navigateTo('dashboard');
+        }
+      });
     }
 
     // Send / Stop

wwwroot/js/router.js

@@ -46,7 +46,13 @@ async function navigateTo(page, params = {}, options = {}) {
 
   // Back-Buttons verdrahten
   document.querySelectorAll('.back-btn[data-page]').forEach(btn => {
-    btn.addEventListener('click', () => history.back());
+    btn.addEventListener('click', () => {
+      if (history.state && history.length > 2) {
+        history.back();
+      } else {
+        navigateTo(btn.dataset.page || 'dashboard');
+      }
+    });
   });
 }