feat: show the transcript while speakers are still being identified (#134)

[raphaelm22]

Closes #134.

Summary

With pyannote diarization on, the transcript stayed hidden until diarization finished. That can take about as long as the recording on a CPU: 34 minutes for a 31-minute meeting whose transcription took 5.

The transcript is now displayed and saved as soon as transcription finishes. Speaker labels fill in when diarization completes.

• _start_diarization calls the new _show_transcript_preview once the worker has started:
  • the displayed session gets display_transcript plus _save_transcript, and the progress bar stays up;
  • any other session gets _write_transcript_for_session.
• Edits are kept. The preview shares its segment objects with DiarizationWorker, which only ever sets .speaker. Text edited while diarization runs is already in the final result.
• Reopened previews: if the user switches to another recording and back, the preview is reloaded from disk and is no longer the worker's object. _finish_diarized_transcript copies its edits onto the final result before displaying it, on both the finished and error paths. Segments are matched by position and timestamps; anything else is treated as a different transcript and left alone.
• Progress bar on return: reopening a recording whose speakers are still being identified shows the progress bar again.
• Unchanged: summaries, chat context, the tray notification and the job queue still wait for _display_final_transcript. Auto-summary needs the speakers, and the queue is still busy.
• Crash safety, as a side effect: the transcript is on disk before diarization starts, so closing the app mid-diarization no longer loses it.

Tests

tests/test_transcript_preview.py, 17 new tests, using the stub-self pattern from test_system_monitor_start.py. DiarizationWorker is patched with autospec. The tests cover:

• the preview follows the worker start;
• no preview when there is no audio;
• display and save for the current session, and its own directory for another;
• the preview leaves the progress bar, auto-summary and the queue alone;
• _carry_over_edits: the edit reaches the final result, and same object, other timestamps, other length and nothing shown are all ignored;
• both diarization outcomes go through _finish_diarized_transcript;
• the progress bar shows only for the recording being diarized, and only while it runs.

Without the change, 14 of the 17 fail. Full suite: 425 passed.

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