[Feature]: Import SRT subtitles into the transcript #1365

Today the transcript (and the captions built from it) can only come from OpenScreen's own transcription. Users who already have subtitles, from a human transcriber or another tool, can't bring them in.

Importing an .srt file would cover that. The UX needs to be designed before any code:

• Where the action lives (transcript pane, captions, media page).
• What happens to an existing transcript: replace, or refuse.
• Whether the file's timestamps refer to the source recording or to the edited timeline (after cuts and speed regions).
• How errors in a malformed file are reported.

First explored in #1116, which bundled it with other changes.