Captions: File ▸ Export ▸ Captions… asks for the file format (#3365) (#3523)

The menu item, and the Text panel's Export captions…, opens Caption Export
Settings with a File Format menu of every format captions.export writes. OK
opens the save panel with only that format's filter and a matching name. A
name typed with another caption format's extension is written in that format
(it is the file the save panel asked about); any other name gets the chosen
format's extension. If a file with that new name already exists, nothing is
written: with WebVTT chosen, typing "keep" when keep.vtt exists leaves
keep.vtt alone. With a path the command still exports directly.

Also fixes a panic: captions.export with params that are not an object
indexed into them.

Fixes #3365

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

Cargo.lock

@@ -2494,6 +2494,7 @@ dependencies = [
  "egui",
  "egui_kittest",
  "filmcraft-audio-dsp",
+ "filmcraft-captions",
  "filmcraft-codecs",
  "filmcraft-color",
  "filmcraft-edit",