fix(export): skip redundant AAC re-encode during final mux

[meiiie]

Summary

• avoid re-encoding source audio during final FFmpeg mux when the export is in the safe copy-source path and the source audio is already AAC-compatible
• thread source audio codec metadata from the exporters to the native/fallback mux boundary
• add focused unit coverage for the copy-vs-transcode decision logic

Root Cause

The final FFmpeg mux path always re-encoded audio to AAC, even when the export was simply reattaching an unedited source audio track that was already AAC-compatible for MP4. That extra transcode work made the FFmpeg subprocess the long pole near 99% finalization.

Validation

• node .\\node_modules\\typescript\\bin\\tsc --noEmit
• node .\\node_modules\\vitest\\vitest.mjs run electron/ipc/nativeVideoExport.test.ts
• node .\\node_modules\\@biomejs\\biome\\bin\\biome check electron/electron-env.d.ts electron/preload.ts electron/ipc/nativeVideoExport.ts electron/ipc/export/native-video.ts src/lib/exporter/modernVideoExporter.ts src/lib/exporter/videoExporter.ts electron/ipc/nativeVideoExport.test.ts
• local FFmpeg A/B benchmark on recording-1776619033302.mp4:
  • -c:a copy: average 202.42 ms
  • -c:a aac -b:a 192k: average 634.17 ms

Issues

• Closes part of export time out #221
• Closes part of Export stuck 99% #194

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[meiiie]

Closing this draft to keep the export queue focused on the better-validated edited-track follow-up in #297. The copy-source fast path still looks directionally useful, but I do not have enough end-to-end validation to keep it open as a separate PR right now.