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/dashboard.html

@@ -551,6 +551,11 @@
                         <textarea id="fb-message" required aria-required="true" minlength="10" maxlength="5000" rows="6" placeholder="Beschreibe dein Anliegen (mind. 10 Zeichen)..."></textarea>
                         <div class="form-hint"><span id="fb-char-count">0</span> / 5.000 Zeichen</div>
                     </div>
+                    <div class="form-group">
+                        <label for="fb-files">Bilder anhaengen (optional)</label>
+                        <input type="file" id="fb-files" accept="image/jpeg,image/png" multiple style="font-size:13px;">
+                        <div class="form-hint">Max. 3 Bilder (JPEG/PNG, je max. 5 MB)</div>
+                    </div>
                 </div>
                 <div class="modal-footer">
                     <button type="button" class="btn btn-secondary" onclick="closeModal('modal-feedback')">Abbrechen</button>