Save the transcript as subtitles too, not just as text

whisper-1's verbose response carries a start and an end for every segment,
and the file tab was reading only the start, to build the [mm:ss] prefix.
Keeping the end as well is all an SRT needs.

The text stays the authority on wording and the segments on timing; they
meet at that prefix, which the cleanup model is already told to leave alone.
So a transcript that went through cleanup still turns into properly timed
subtitles. A line whose stamp matches no segment runs until the next line
starts, a line with no stamp at all joins the cue above it, and an end that
would run into the next cue is trimmed back.

The button is dead until a timestamped run finishes, because without
timestamps there are no segments to time anything with.

README.md

@@ -80,7 +80,8 @@ elapsed time, then the stage it is on. It never takes focus. Pressing
   dictation is not lost, but the indicator turns amber with the reason instead of
   looking like a normal run.
 - **Audio and video files** run through the same models under Settings → Audio
-  file, optionally with `[mm:ss]` timestamps, chunked through ffmpeg when long.
+  file, optionally with `[mm:ss]` timestamps, chunked through ffmpeg when long,
+  and saved as `.txt` or as `.srt` subtitles.
 - **History** of every dictation under Settings → History, with a size limit and
   right-click to delete.
 - **Turkish and English interface**, following the system locale by default.