feat(desktop): live ffmpeg encoder progress events for long video exports

[huntharo]

Summary

Stream progress updates from the ffmpeg encoder to the renderer over EVENT_CHANNELS.renderProgress so users get a progress bar / percentage on long GIF or MP4 exports instead of a spinner of unknown duration.

Listed in PR #66's notable follow-ups; not addressed in the merged scope.

Current state

recording-exporter.ts:185 runFfmpeg shells out and awaits exit. No stderr parsing, no incremental updates. The renderer shows the export button in a generic "Encoding…" state with no indication of how far along it is.

Short clips finish before the user notices; long ones (30s+ GIF with palettegen, or a multi-minute MP4 export) leave the user wondering whether anything's happening.

Scope

• Parse ffmpeg's stderr time= / out_time_ms= lines (or use -progress pipe:1 for cleaner machine-readable output).
• Compute a percentage against the known input duration (we have it from video_captures.duration_sec).
• Emit renderProgress events on a throttled cadence (e.g. every 250ms).
• Renderer's export button state in FloatOver.tsx shows a percentage / progress ring instead of just "Encoding…".
• Make sure progress events don't survive the export — clear them in done / error final states.

Out of scope

• Pause/cancel of in-flight exports. Different problem (need to manage the child process lifecycle and handle partial output).
• Multi-step pipelines (e.g. GIF's palettegen → paletteuse). Report combined progress as best-effort; a coarse two-phase split is fine.

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