feat: implemented transcript editor

[abhalsod-sonata]

Description

Implements the In-Platform Transcript Editor for the Videos page in Studio
(frontend-app-authoring). Course authors can now edit and auto-save SRT
transcripts directly within Studio - without downloading, editing locally,
and re-uploading.

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Demo - Screen Recording

2026-05-14.13-35-11.mp4

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Testing

Manual

• Open Videos page (List view) → transcript count shown as a
  clickable link → clicking opens the info modal
• In info modal → click ⋯ → Edit transcript → transcript
  editor opens
• Edit transcript cue text → "Saving…" spinner appears →
  "✅ Saved" shown on success
• Simulate save failure → error shown in editor header
• Click + Add a transcript → language dropdown and Upload
  file button shown
• Upload a valid .srt file → file confirmed → "Add transcript"
  button enables → transcript count updates → success toast appears
• Upload a file > 25MB → inline size error shown before upload
• Upload an invalid (non-SRT) file → inline format error shown
• Toggle enableTranscriptEditor waffle flag OFF →
  "Edit transcript" option disappears from ⋯ menu
• Verify existing Download, Replace, Delete transcript flows
  are unaffected
• Open Videos page (Grid view) → ⋯ → Info and transcripts
  menu label is correct

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Waffle Flag

enableTranscriptEditor (default: false)

Registered in src/data/api.ts → waffleFlagDefaults.
Enable via Django Admin or MFE config to expose the editor UI.

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New API

GET /transcripts/?edx_video_id={videoId}&language_code={language}

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Backend PR:

• PR : feat: Add transcript editor toggle and update related serializers and tests edx-platform#267

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Jira ticket:

• https://2u-internal.atlassian.net/browse/TNL2-515