fix(screen_recorder): record the screen, not the webcam, on macOS

avfoundation enumerates cameras before screens, so passing screen_index
straight through to -i selects device 0 — the built-in camera on any Mac
that has one. The tool then silently records the user's face while the
caller believes it is capturing the screen, with no error to reveal it.

That is a privacy problem as much as a correctness one:
screen_capture_selector advertises this tool as "no webcam" (and routes
webcam overlay work to cap_recorder), and screen_index is documented as a
monitor index, not a device index. The Windows and Linux branches do not
take screen_index at all, so the mac branch was the only one leaking a
platform device numbering into a cross-platform parameter.

Map the monitor index to its real avfoundation device index by parsing
-list_devices, falling back to the raw index when detection fails.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GYThSL15CujvBwD1wuUmr9

Author: hubooy

tools/capture/screen_recorder.py

@@ -54,6 +54,28 @@ def _detect_audio_device_windows() -> str | None:
     return None
 
 
+def _detect_screen_device_mac(screen_index: int) -> str | None:
+    """Map a monitor index to its avfoundation device index.
+
+    avfoundation numbers cameras before screens, so device 0 is a webcam on
+    any Mac with a camera — passing the monitor index straight through would
+    silently record the camera instead of the screen.
+    """
+    try:
+        result = subprocess.run(
+            ["ffmpeg", "-f", "avfoundation", "-list_devices", "true", "-i", ""],
+            capture_output=True, text=True, timeout=10,
+        )
+        for line in result.stderr.splitlines():
+            if f"] Capture screen {screen_index}" in line and "[" in line:
+                idx = line.split("[")[-1].split("]")[0].strip()
+                if idx.isdigit():
+                    return idx
+    except (subprocess.TimeoutExpired, FileNotFoundError, IndexError):
+        pass
+    return None
+
+
 def _detect_audio_device_mac() -> str | None:
     """Find the default audio input index on macOS via avfoundation."""
     try:
@@ -328,12 +350,14 @@ def _build_mac_cmd(
         cmd += ["-framerate", str(fps)]
         cmd += ["-t", str(duration)]
 
+        screen_device = _detect_screen_device_mac(screen_index) or str(screen_index)
+
         if region:
             # avfoundation doesn't support region directly — we crop in post
-            cmd += ["-i", f"{screen_index}:{audio_idx}"]
+            cmd += ["-i", f"{screen_device}:{audio_idx}"]
             cmd += ["-vf", f"crop={region['width']}:{region['height']}:{region.get('x', 0)}:{region.get('y', 0)}"]
         else:
-            cmd += ["-i", f"{screen_index}:{audio_idx}"]
+            cmd += ["-i", f"{screen_device}:{audio_idx}"]
 
         cmd += ["-c:v", "libx264", "-preset", "ultrafast", "-crf", "23"]
         if capture_audio and audio_idx != "none":