Fix missing documentation redirects in Docker frontend build

[LunaStev]

The frontend imports wavedoc/redirects.json, but the frontend-builder Docker stage only copied the editor and frontend trees. A clean Docker build therefore failed with TS2307 even though building from a complete checkout passed.

Copy the shared JSON into the frontend stage before compilation. Add a CI build of that exact Docker stage before other steps generate local dependencies or artifacts, so missing build inputs cannot pass unnoticed again.

Verification

• Reproduce TS2307 with the unchanged upstream/master Dockerfile.
• Build the corrected frontend-builder stage successfully.
• Run all 4 documentation navigation tests inside the resulting image.
• git diff --check.
• PR CI, including the Docker frontend stage and existing editor/frontend checks.