feat: admin users can view all downloads with user badges

- Admin users (Emby IsAdministrator) see a 'Show all users' toggle
- When toggled, all tagged downloads are shown regardless of user
- Each download card shows the tagged user's name as a badge
- Non-admin users see only their own downloads (unchanged behavior)
- Backend accepts ?showAll=true query param (admin-only)

public/index.html

@@ -46,6 +46,12 @@
                             <option value="0">Off</option>
                         </select>
                     </div>
+                    <div id="admin-controls" class="admin-controls" style="display: none;">
+                        <label class="toggle-label">
+                            <input type="checkbox" id="show-all-toggle">
+                            <span>Show all users</span>
+                        </label>
+                    </div>
                     <div class="user-info">
                         <span class="user-label">Current User:</span>
                         <span class="user-name" id="currentUser">-</span>