WIP mp4 playback Linux: add gstreamer backend for mp4 playback

[alltheseas]

Title: Add GStreamer-backed MP4 playback with pause/resume fixes

• add new notedeck_media_gst crate wired into AppContext, expose VideoManager types, and enable inline/
  fullscreen playback of HTTP(S) MP4s.
• bundle the GStreamer dependencies (gstreamer, gstreamer-app, gstreamer-video, etc.) and thread the manager
  pipeline, poster extraction, frame uploads, and event propagation into egui.
• harden the backend to avoid panics when playbin lacks a user-agent property, survive flush/pause paths by
  translating BoolError into FlowError::Flushing, and add frame-level tracing so pauses/resumes no longer
  freeze the video surface.
• adjust timeline/thread.rs to fix an Into ambiguity introduced by the new deps.

assessment

GStreamer is not very performant. Can only run on software (disableing hardware acceleration) on Linux.

#248

[alltheseas]

superseded by egui-video-rs player #1172