Preserve recording timestamps through file transcription and export

[RioPlay]

Recorded-file transcription previously collapsed track offsets and timestamp gaps. This draft adds recording-clock transcription and SRT/VTT export: a late track stays late, and an internal pause remains in the subtitle timeline. The window inspects actual audio streams, offers Keep recording timestamps when a common clock exists, and explains when only relative timing is available. Changing track or timing clears the old preview; unchanged reinspection preserves it. CLI/API callers explicitly select recording timing and retain relative defaults. Ordinary PCM WAV still works without extra tools.

An explicitly selected compatible FFmpeg/FFprobe pair supplies original frame timing through a bounded private metadata journal and independently flushed resampling runs. Source/tool checks, exact sample counts, successful child exits and journal integrity are required before a transcript can complete. Late failures discard tentative recognition. There is no automatic executable discovery, model download, recording or export. Decoder setup stays compact with scrolling guidance and separate verified tool selections. Source PyAV remains separate from packaged support.

Validation: 309 targeted tests passed with no skips, including eight real selected-tool cases. A real two-track MKV reaches production transcription and atomic SRT/VTT export with only the speech engine substituted: its 1-second origin normalizes once; second-track cues remain at 0.2 and 0.4 seconds, while explicit relative mode starts at zero. Independent backend and UI/CLI reviews pass. Seven synthetic Windows captures cover default/compact file and setup windows; current screenshots are included in documentation. All five desktop jobs passed in CI 34788391220 at 770ebf0. The preceding adapter checkpoint babaae6 passed all five desktop jobs in CI 34787584275.

Grouped multi-track jobs/export and remaining recording/release acceptance stay open in docs/plans/active/recorded-file-timelines.md. This is unreleased source; it does not establish general speech quality or live OBS acceptance. Published desktop RC2 and Android alpha15 remain separate. Stacked on #41 at 813cce7.