feat: add WebRTC streaming via str0m + portal session persistence

- Add src/webrtc.rs: HTTP signaling server + str0m Sans-IO WebRTC transport
  with H.264 Annex-B → RTP packetization and key-frame request handling
- avhw: introduce FrameOutput enum (Muxer | Channel) so SwEncState can
  output to either MP4 muxer or crossbeam channel for WebRTC
- cap_portal: support portal session restore tokens (PersistMode::ExplicitlyRevoked)
  to skip re-authorization dialog; add --no-persist flag to force fresh dialog
- args: make --output optional when --port is used for WebRTC mode
- state_portal: integrate WebRTC pipeline (encoder channel → RTP forwarding)
  with shorter GOP for WebRTC (fps/2, min 10)
- main: redirect tracing to stderr; validate --output or --port required
- Add dependencies: str0m 0.20, serde_json 1, dirs 6

src/state.rs

@@ -613,7 +613,7 @@ impl<S: CaptureSource> State<S> {
             .unwrap_or_else(|| 2 * (width as u64) * (height as u64) * (fps as u64) / 100);
         let enc = match crate::avhw::create_encoder(
             &drm_path,
-            Path::new(&self.args.output),
+            Path::new(self.args.output.as_deref().expect("output required for MP4 mode")),
             width,
             height,
             fps,