fix: recording produces correct video duration with real-time ffmpeg encoding (#812)

* fix: replace screenshot polling with screencast-based piped ffmpeg recording

  Recording previously used Page.captureScreenshot polling at 10fps,
  which was CPU-heavy and produced inconsistent results. Now uses
  Page.startScreencast with throttled acks (35ms interval) to receive
  frames event-driven from Chrome, and pipes JPEG data directly to
  ffmpeg stdin in real-time instead of saving temp files.

  - Spawn ffmpeg at recording start with piped stdin (image2pipe)
  - Background task receives screencast frames, interpolates gaps by
    repeating the last frame based on timestamps, targets 25fps
  - Ack throttling controls Chrome's frame push rate
  - Fix: current frame was never written after the first one
  - Fix: frame count was read before task finished padding
  - Remove tokio-util dependency (replaced CancellationToken with oneshot)
  - Add tokio "process" feature for async child process stdin pipe
  - Extract start/stop_recording_task helpers on DaemonState
  - Add tests for restart, ffmpeg codec selection, and stop without task

* fmt

* chore

* fix: switch WebM codec from VP9 to VP8 for correct framerate and browser
  compatibility

  VP9 realtime encoder ignored input framerate, producing 10fps output
  instead of 25fps. This caused inconsistent playback in browsers.
  VP8 respects -framerate 25 and has wider browser playback support.

* fmt

* fix: add kill_on_drop to ffmpeg process to prevent zombie on task panic

* fix: switch from screencast to screenshot polling for reliable recording duration

  Screencast only pushes frames on visual changes, producing short videos
  on static pages. Screenshot polling captures at a fixed 10fps interval
  regardless of page activity, guaranteeing duration matches wall-clock time.
  ffmpeg piped stdin architecture is preserved — no temp files.

---------

Co-authored-by: hyunjinee <leehj0110@kakao.com>

cli/Cargo.toml

@@ -16,7 +16,7 @@ serde_json = "1.0"
 dirs = "5.0"
 base64 = "0.22"
 getrandom = "0.2"
-tokio = { version = "1", features = ["rt-multi-thread", "macros", "net", "io-util", "time", "sync", "signal"] }
+tokio = { version = "1", features = ["rt-multi-thread", "macros", "net", "io-util", "time", "sync", "signal", "process"] }
 tokio-tungstenite = { version = "0.24", features = ["rustls-tls-webpki-roots"] }
 futures-util = "0.3"
 url = "2"