Add screenshot consent notice and improve video frame/audio handling

- add required dashboard screenshot consent/privacy notice to README
- update copilot instructions to preserve consent notice in docs
- document external VLC audio behavior (`--no-audio` for muted/0%, `--gain` otherwise)
- capture real video frames for dashboard screenshots instead of black compositor output
- make ffmpeg fallback frame capture playback-relative (loop-aware) to avoid repeated stale image

.github/copilot-instructions.md

@@ -9,6 +9,7 @@
 - ✅ **Virtual environment MUST have** pygame + pillow (required for Impressive)
 - ✅ **Client-side resize/compress** screenshots before MQTT transmission
 - ✅ **Server renders PPTX → PDF via Gotenberg** (client only displays PDFs, no LibreOffice needed)
+- ✅ **Keep screenshot consent notice in docs** when describing dashboard screenshot feature
 
 ### Key Files & Locations
 - **Display logic**: `src/display_manager.py` (controls presentations/video/web)
@@ -261,6 +262,8 @@ FILE_SERVER_SCHEME=http          # http or https
 - **Fields**: `url`, `autoplay` (bool), `loop` (bool), `volume` (0.0-1.0 → 0-100)
 - **URL rewriting**: `server` host → configured file server
 - **Fullscreen**: enforced for python-vlc on startup (with short retry toggles); external fallback uses `--fullscreen`
+- **External VLC audio**: `muted=true` (or effective volume 0%) starts with `--no-audio`; otherwise startup loudness is applied via `--gain=<0.00-1.00>`
+- **Runtime volume semantics**: python-vlc supports live updates; external VLC fallback is startup-parameter based
 - **Monitoring PID semantics**: python-vlc runs in-process, so PID is `display_manager.py` runtime PID; external fallback uses external `vlc` PID
 - **HW decode errors**: `h264_v4l2m2m` failures are normal if V4L2 M2M unavailable; use software decode
 - Robust payload parsing with fallbacks
@@ -298,6 +301,8 @@ FILE_SERVER_SCHEME=http          # http or https
 
 - The Display Manager now prefers using python-vlc (libvlc) when available for video playback. This enables programmatic control (autoplay, loop, volume) and cleaner termination/cleanup. If python-vlc is not available, the external `vlc` binary is used as a fallback.
 - Supported video event fields: `url`, `autoplay` (boolean), `loop` (boolean), `volume` (float 0.0-1.0). The manager converts `volume` to VLC's 0-100 scale.
+- External VLC fallback applies audio at startup: `--no-audio` when muted/effective 0%, otherwise `--gain` from effective volume.
+- Live volume adjustments are reliable in python-vlc mode; external VLC fallback uses startup parameters and should be treated as static per launch.
 - URLs using the placeholder host `server` (for example `http://server:8000/...`) are rewritten to the configured file server before playback. The resolution priority is: `FILE_SERVER_BASE_URL` > `FILE_SERVER_HOST` (or `MQTT_BROKER`) + `FILE_SERVER_PORT` + `FILE_SERVER_SCHEME`.
 - Hardware-accelerated decoding errors (e.g., `h264_v4l2m2m`) may appear when the platform does not expose a V4L2 M2M device. To avoid these errors the Display Manager can be configured to disable hw-decoding (see README env var `VLC_HW_ACCEL`). By default the manager will attempt hw-acceleration when libvlc supports it.
 - Fullscreen / kiosk: the manager will attempt to make libVLC windows fullscreen (remove decorations) when using python-vlc, and the README contains recommended system-level kiosk/session setup for a truly panel-free fullscreen experience.