feat(agent): add screen blackout feature

- Physical displays blanked via overlay
- Remote video and input keep working
- Exclude overlay from screen capture
- Add blackout API and UI toggle
- Wire flag through capture protocol

lib/agent/web/index.html

@@ -819,7 +819,7 @@
         <div class="panel-head">
           <div>
             <h2>Remote desktop</h2>
-            <p class="lede">Live monitor stream. Interaction is off until you enable it.</p>
+            <p class="lede">Live monitor stream. Interaction is off until you enable it. Black out screen blanks the physical displays; remote view and control keep working.</p>
           </div>
           <div class="panel-actions">
             <button id="video-start" class="primary" type="button">Start</button>
@@ -841,6 +841,10 @@
             <input id="video-interact" type="checkbox">
             Interactable
           </label>
+          <label class="check">
+            <input id="video-blackout" type="checkbox">
+            Black out screen
+          </label>
         </div>
         <div class="stage">
           <canvas id="video-canvas" class="view-only" width="1280" height="720"></canvas>