feat(captions): export subtitle files

[meiiie]

Description

Add separate subtitle-file export for generated captions, with SRT and WebVTT output options in the Captions panel.

Motivation

Issue #334 asks for exporting captions/subtitles as a separate file rather than baking captions into the rendered video. This keeps the feature scoped to caption-file export and avoids coupling it to the video render pipeline.

Type of Change

• Bug Fix
• New Feature
• Refactor / Code Cleanup
• Documentation Update
• Other (please specify)

Related Issue(s)

• Feature Request: Export subtitles as a separate file #334

Changes Made

• add SRT and WebVTT serializers for existing caption cues
• add focused serializer coverage for timestamps, cue ordering, whitespace cleanup, and invalid cue filtering
• add Export SRT and Export VTT actions to the Captions panel when captions exist
• add a dedicated Electron save path for subtitle files with subtitle-specific filters and UTF-8 output

Testing Guide

1. Generate captions in the editor.
2. Open the Captions panel.
3. Click Export SRT and save the file.
4. Click Export VTT and save the file.
5. Verify each file opens as text and contains the expected timestamps and caption text.
6. Verify the export buttons are disabled when no captions exist.

Checklist

• I have performed a self-review of my code.
• I have tested the changes locally.
• I have added focused regression coverage for the new behavior.
• I have completed a manual Electron save-dialog smoke test.
• I have linked screenshots or videos where helpful.

Local checks run:

• vitest run src/components/video-editor/subtitleExport.test.ts
• tsc --noEmit
• biome check --formatter-enabled=false electron/ipc/register/export.ts electron/preload.ts electron/electron-env.d.ts src/components/video-editor/SettingsPanel.tsx src/components/video-editor/VideoEditor.tsx src/components/video-editor/subtitleExport.ts src/components/video-editor/subtitleExport.test.ts

Notes:

• The scoped Biome check still reports the existing handleExport smoke-export dependency warnings in VideoEditor.tsx; this PR leaves those unrelated warnings untouched.

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[meiiie]

Closing this old draft for now so the open PR list reflects current release work. Subtitle-file export can be revived later as a fresh PR from current main with an Electron save-dialog smoke and focused owner review.