Add image attachments to feedback form (JPEG/PNG)

- File input in feedback modal (max 3 images, 5 MB each)
- Frontend validation for file count and size
- Backend: multipart/form-data with UploadFile, MIME attachments
- Images attached to feedback email as base64-encoded attachments
- Only JPEG and PNG allowed (validated server-side)

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

src/static/js/app.js

@@ -2168,10 +2168,30 @@ const App = {
             return;
         }
 
+        // Dateien pruefen
+        const fileInput = document.getElementById('fb-files');
+        const files = fileInput ? Array.from(fileInput.files) : [];
+        if (files.length > 3) {
+            UI.showToast('Maximal 3 Bilder erlaubt.', 'error');
+            return;
+        }
+        for (const f of files) {
+            if (f.size > 5 * 1024 * 1024) {
+                UI.showToast('Datei "' + f.name + '" ist groesser als 5 MB.', 'error');
+                return;
+            }
+        }
+
         btn.disabled = true;
         btn.textContent = 'Wird gesendet...';
         try {
-            await API.sendFeedback({ category, message });
+            const formData = new FormData();
+            formData.append('category', category);
+            formData.append('message', message);
+            for (const f of files) {
+                formData.append('files', f);
+            }
+            await API.sendFeedbackForm(formData);
             closeModal('modal-feedback');
             UI.showToast('Feedback gesendet. Vielen Dank!', 'success');
         } catch (err) {