fix(model-manager): replace window.confirm with in-app modal

Browsers suppress native confirm() dialogs after repeated use (the
"prevent this page from creating additional dialogs" checkbox), which
silently broke deletes. Add a promise-based in-app confirmation modal
and use it for gallery photo and installed-model deletes.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

model-manager/app/static/index.html

@@ -194,6 +194,17 @@
     </div>
   </div>
 
+  <!-- In-app confirmation dialog (replaces native window.confirm) -->
+  <div id="confirmModal" class="modal-overlay" hidden>
+    <div class="modal">
+      <p id="confirmMsg"></p>
+      <div class="modal-actions">
+        <button class="btn" id="confirmCancel">Cancel</button>
+        <button class="btn danger-solid" id="confirmOk">Delete</button>
+      </div>
+    </div>
+  </div>
+
   <script src="/app.js"></script>
 </body>
 </html>