feat: add admin-only status page with cache stats

- New /api/dashboard/status endpoint (admin-only, 403 for non-admins)
- Returns server info (uptime, Node version, memory usage)
- Returns polling mode and interval
- Returns cache stats: entry count, total size, per-key breakdown
  with item count, size in KB, and TTL remaining
- Status button in admin controls header
- Collapsible status panel with grid layout
- Responsive: single column on mobile

public/index.html

@@ -57,6 +57,7 @@
                             <input type="checkbox" id="show-all-toggle">
                             <span>Show all users</span>
                         </label>
+                        <button id="status-btn" class="status-btn">Status</button>
                     </div>
                     <div class="user-info">
                         <span class="user-label">Current User:</span>
@@ -66,6 +67,8 @@
                 </div>
             </header>
 
+            <div id="status-panel" class="status-panel" style="display: none;"></div>
+
             <div id="error-message" class="error-message" style="display: none;"></div>
 
             <div id="loading" class="loading" style="display: none;">Loading downloads...</div>