Add footer asset selector for email customization

- New /api/assets endpoint to list available icons from assets folder
- Dynamic footer generation with multiple selectable icons
- Thumbnail grid UI for asset selection in the mail form
- Selected icons are embedded as CID attachments in emails

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

public/index.html

@@ -43,6 +43,13 @@
           <textarea id="body" name="body" required rows="8" placeholder="Ihre Nachricht..."></textarea>
         </div>
 
+        <div class="form-group">
+          <label>Footer-Icons</label>
+          <div id="assetGrid" class="asset-grid">
+            <p class="loading-message">Lade Assets...</p>
+          </div>
+        </div>
+
         <button type="submit" id="submitBtn">Senden</button>
       </form>