Export progress + cancel UI — ED.22

Striking a print took minutes, and the lab tech didn't just walk away — they watched the footage counter climb and could halt a bad run before it wasted the whole reel. ED.22 is that counter and that stop button: an ExportBar that turns ED.21's export into a visible, interruptible job.

sequenceDiagram
  participant UI as ExportBar (Leptos)
  participant CMD as editor_export (Tauri)
  participant GEN as export_edited_project
  UI->>CMD: invoke(project, "mp4")
  CMD->>GEN: spawn_blocking(...)
  loop each frame
    GEN-->>CMD: on_progress(done, total)
    CMD-->>UI: emit editor-export-progress
  end
  UI->>CMD: editor_export_cancel (Cancel)
  CMD-->>GEN: cancel flag set
  GEN-->>UI: editor-export-done { path } / -error

The backend editor_export command runs the export on the blocking pool (so the webview stays live), deriving the output path beside the recordings folder and emitting editor-export-progress throttled to ~100 events over the run. A shared AtomicBool (raised by editor_export_cancel) is the stop button — the export loop polls it each frame. The progress / done / error events ride the same __TAURI__.event → CustomEvent → signal bridge the rest of the editor uses, landing in an ExportUiState the bar renders: Export when idle, a progress bar + Cancel while running, and the output path (or error) when done.