feat: add in-platform transcript editor to the videos page

[abhalsod-sonata]

Adds an in-platform transcript editor to the Studio videos page. Course authors can
open "Edit transcript" from a transcript row's action menu and edit the SRT in the
browser instead of the download → edit locally → re-upload round-trip.

What's included:

• New transcript-editor/ module: cue-by-cue text editing with SRT parsing/validation
  (srtUtils), invalid-SRT alerting, debounced auto-save, and a video preview with
  click-to-seek + active-cue highlighting and auto-scroll.
• The video Info content moves under the preview (InfoTab via a new
  infoModalContentUnderPreview slot on FileTable/InfoModal), and the info sidebar
  lists transcripts directly - this intentionally supersedes the previous tabbed
  sidebar layout for videos.
• The row action menu label for videos changes from "Info" to "Info and
  transcripts" (FileMenu, MoreInfoColumn); two existing tests that asserted the
  old label on video fixtures are updated.
• Saving a transcript re-uploads the SRT through the existing /transcript_upload/
  endpoint; the UI distinguishes replace (200) from create (201) - see dependency
  below.

Impacted user roles: Course Authors (primary); Developers (new
transcript-editor module and the infoModalContentUnderPreview extension point on
the generic file table).

Screen recording:

• Before:

Before.mp4

• After

After.mp4

Supporting information

• Depends on feat: return 200 when a transcript upload replaces an existing language openedx-platform#39038 (200-on-replace for
  /transcript_upload/) - please merge that first; without it the editor cannot
  distinguish replace from create.
• Product review for this feature (approved; this repo's maintainer agreed to
  maintain it)
• Internal (private, 2U Jira - context restated in this description): LP-910 under
  epic LP-735. Originally shipped downstream as feat: implemented transcript editor edx/frontend-app-authoring#97 + test: add test for AppCard #98;
  ported to current master with the adaptations listed under "Other information".
  The downstream feature flag was removed on purpose (product decision) - the feature
  ships enabled.

Testing instructions

Automated:

npm ci
npx jest src/files-and-videos --coverage=false
# 20 suites / 229 tests expected green

Manual (tutor dev):

1. Check out feat: return 200 when a transcript upload replaces an existing language openedx-platform#39038 in your platform (or any branch
   containing it) and run Studio.
2. npm run dev in this repo (port 2001); open a course with videos that have
   transcripts → Videos page.
3. Row ⋮ menu shows "Info and transcripts" (videos only; Files page unchanged);
   the info modal shows the video preview with Info content below it and
   transcripts listed in the sidebar.
4. Transcript row menu → Edit transcript: edit a cue, watch auto-save, Save;
   reopen to confirm persistence; click cues to seek the preview.
5. Paste invalid SRT content → validation alert, save blocked.
6. Network tab: re-saving an existing language returns 200; adding a new language
   returns 201.

Other information

• Port provenance & adaptations (for reviewers comparing against the downstream
  PRs): rebased onto current master, which required - merging with the new
  permissions gating in FileTable/MoreInfoColumn; porting one-line changes into
  the TS-renamed FileMenu.tsx and messages.ts files; re-applying the
  injectIntl → useIntl migration on top of the reworked Transcript.jsx; and
  keeping upstream's key={idx} fix in TranscriptTab.
• CourseVideosTable still passes activeTab/setActiveTab into the sidebar factory
  for compatibility; the new sidebar ignores them. Happy to remove that plumbing here
  or in a follow-up if preferred.
• Playback: seekTo() guards the play() promise and ignores AbortError
  (interrupted play on rapid cue-seek/pause/unmount), logging anything else.
• Accessibility: built from Paragon components (Menu, AlertModal, form controls);
  no dedicated audit yet - feedback welcome, especially on cue-list keyboard
  navigation.
• i18n: all new messages include translator descriptions; existing message ids are
  unchanged so downstream translations carry over.

Best Practices Checklist

• Any new files are using TypeScript (.ts, .tsx).
  - The new transcript-editor/ module is ported as .jsx/.js to stay
  reviewably close to the downstream original. Happy to convert
  TranscriptEditor/srtUtils to TS in this PR if preferred - say the word.
• Avoid propTypes and defaultProps in any new or modified code.
  - Ported/modified components follow the surrounding files' existing
  PropTypes style; will drop them as part of the TS conversion above if requested.
• Tests should use the helpers in src/testUtils.tsx (initializeMocks)
  - Changes extend the existing videos-page suites, which predate
  initializeMocks; kept their established setup for consistency.
• Do not add new fields to the Redux state/store. (No slice/thunk changes;
  editor state is component-local.)
• Use React Query to load data from REST APIs.
  - Transcript content is fetched via the page's existing axios helpers
  (videos-page/data/api.js); this page has not been migrated to React Query yet.
• All new i18n messages in messages files have a description for translators.
• Avoid ../ in import paths; use @src for parent-folder imports.

Relevant backend PR

• feat: return 200 when a transcript upload replaces an existing language openedx-platform#39038

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[mphilbrick211]

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[abhalsod-sonata]

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[abhalsod-sonata]

@bradenmacdonald
Can you please review and approve this PR?