fix(render): abort child processes and prevent partial outputs on cancel

[ubugeeei]

Finding

Cancel/error paths can still progress into concat/mux/finalization, which can publish partial outputs as the normal output file.

Evidence

• render/src/main.rs polls cancel state while worker/ffmpeg phases continue.
• render/src/ffmpeg.rs manages long-running child processes.

Acceptance criteria

• Propagate cancellation to child processes.
• Write to a temporary output and atomically promote only on success.
• Leave a clear failed/canceled status for the UI.

[ubugeeei]

対応済みです。修正は PR #1 / commit 9cf4498 に反映済みです。