TTS window: OK applies subtitle changes to main window, Cancel discards

[niksedk]

Implements the third item of cvrle77's report in #12093: changes made in the TTS review should be applied to the main subtitle.

Before

• The window closed with a single Done button that neither distinguished accept from discard nor propagated the session's subtitle changes.
• Review text edits triggered a separate Yes/No prompt after the window closed - regardless of how it was closed.
• Lines merged by the merge-continuation-lines step could not be applied at all; their review edits were only reported as "could not be updated".

After

• Done is replaced by OK and Cancel. OK closes the window accepting the session's subtitle changes; Cancel stops a running generation, or - when idle - closes discarding the changes (exactly what Escape already did, so keyboard and buttons now match).
• On OK the main window applies the changes directly instead of prompting, with a status-bar summary:
  1. Merged lines first - each merged paragraph replaces the run of lines it spans, matched by the time codes (start of the first line, end of the last), then lines are renumbered. If a removed row was selected, selection moves to the merged line.
  2. Review text edits second - matched 1:1 by time codes as before. Because merges apply first, an edit made to a merged sentence in review now applies too, through the merged line's new time codes.
• Title-bar close (X) and Escape count as Cancel.

Also removes the vestigial DoneOrCancelText view-model property (set in four places, bound nowhere) and the now-unused Yes/No prompt language strings; stale keys in existing translation JSONs are ignored by the deserializer. Docs updated.

Note: if the user accepts the merge step but then cancels generation, OK still applies the merges - the merge was explicitly confirmed in its own dialog, so it is treated as an intended subtitle change even without finished audio.

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