[Remove legacy waffle flags] Remove Legacy UI Waffle Flag Consumers — Phase 1c (4 files, most involved)

[farhan]

Part of #521 — Remove Legacy Studio UI Waffle Flags
Frontend ticket: openedx/frontend-app-authoring#3063

/cc @openedx/axim-aximprovements

Scope

Remove routing flag consumers from the four most complex files in `frontend-app-authoring`. These require more involved logic simplification.

Tasks

9. `src/CourseAuthoringContext.tsx`

• 1 flag (`useNewUnitPage`) gating two code paths: `getUnitUrl` (line 69) and `openUnitPage` (line 81)
• Both also check `ENABLE_UNIT_PAGE === 'true'`; collapse to always use `navigate` and the React route
• Remove `useWaffleFlags` call (and import if unused); remove `getConfig` call if the env-var check is also dropped

10. `src/pages-and-resources/pages/PageSettingButton.jsx`

• 2 flags (`useNewTextbooksPage`, `useNewCustomPages`) inside the `determineLinkDestination` memo
• Both branches now always resolve to the new path; simplify to return `/course/${courseId}/${id.replace('_', '-')}` directly for textbooks/tabs
• Remove `useWaffleFlags` call and import

11. `src/editors/containers/VideoEditor/` (two files)

• `components/VideoEditorModal.tsx`: remove `useNewVideoUploadsPage` from the `useWaffleFlags()` destructure (line 45) and from the props passed to `VideoSettingsModal` (line 57)
• `components/VideoSettingsModal/index.tsx`: remove `useNewVideoUploadsPage` from the prop type (line 24), the destructure (line 31), and the conditional render (line 34) — hardcode the upload button to always render when `!isLibrary`

12. `src/header/hooks.tsx`

• 5 flags across 3 functions:
  • `useContentMenuItems`: hardcode outline, updates, files/uploads links to new paths (3 ternaries); simplify the video-upload `if` to always push the item
  • `useSettingMenuItems`: remove `useNewCertificatesPage` guard (line 135) — always push the certificates link
  • `useToolsMenuItems`: hardcode import and export links to new paths (2 ternaries)
• Remove `useWaffleFlags` calls from functions that no longer need routing flags (keep for `enableAuthzCourseAuthoring` and `enableCourseOptimizer`)

Acceptance Criteria

• No references to the removed flag names remain in these 4 files
• `make validate.ci` passes