Follow up Cleanup after removing Legacy UI Waffle Flags

Parent ticket: https://github.com/openedx/frontend-app-authoring/issues/3063

Cleanup Tasks

1. Remove the ENABLE_UNIT_PAGE environment variable flag and all references to it across the codebase, as the unit page is now the default and this flag is no longer needed.
2. Remove the ENABLE_UNIT_PAGE_NEW_DESIGN environment variable flag and all references to it, as the new design is now the standard and the flag has been fully superseded.
3. Remove the ENABLE_NEW_VIDEO_UPLOAD_PAGE environment variable flag and all references to it, as the new video upload page is now the default experience and this toggle is obsolete.
4. Remove isNewHomePage prop from StudioHeader — now that useNewHomePage waffle flag has been removed and the new home page is permanently enabled, the isNewHomePage prop in Header.tsx is hardcoded to true and serves no purpose. This requires a two-step change across two repos:
   • @edx/frontend-component-header — remove the isNewHomePage prop from StudioHeader's interface and destructuring, and hardcode studioBaseUrl: '/home' directly (currently at StudioHeader.tsx:53).
   • frontend-app-authoring (src/header/Header.tsx:104) — once the above package version is published and consumed, drop isNewHomePage from the <StudioHeader> call.

🤖 Generated with Claude Code

Start by searching the frontend-app-authoring codebase for ENABLE_UNIT_PAGE, ENABLE_UNIT_PAGE_NEW_DESIGN, ENABLE_NEW_VIDEO_UPLOAD_PAGE, and related references. Review src/header/Header.tsx:104, then coordinate the @edx/frontend-component-header change at StudioHeader.tsx:53 before removing isNewHomePage from the consumer. Done means all three obsolete flags and references are removed and the header prop is removed in both repositories.