feat(watchdog): separate watchdog from startup

- Add /api/v1/watchdog to enable/disable
- Scheduled task now runs every 15 minutes
- Use schtasks command line instead of XML
- Dashboard shows watchdog enabled state
- Preserve watchdog state during install

lib/agent/web/index.html

@@ -517,6 +517,7 @@
       background: rgba(0, 0, 0, 0.2);
     }
     .startup-card .meta { flex: 1; min-width: 8rem; font-size: 0.92rem; color: var(--text-secondary); }
+    .startup-card + .startup-card { margin-top: 0.65rem; }
 
     /* —— Tables —— */
     .table-wrap {
@@ -757,6 +758,11 @@
           <button id="startup-add" type="button">Add to startup</button>
           <button id="startup-remove" class="danger" type="button">Remove from startup</button>
         </div>
+        <div class="startup-card">
+          <span id="watchdog-state" class="meta">Watchdog: —</span>
+          <button id="watchdog-enable" type="button">Enable Watchdog</button>
+          <button id="watchdog-disable" class="danger" type="button">Disable Watchdog</button>
+        </div>
       </section>
 
       <section id="files" class="panel">