Feat - development server reverse proxy

[chiku-samugari]

Purpose

Companion to osf.io PR #11224.
Enables osf.io (localhost:5000) to act as a reverse proxy for the angular-osf dev server (localhost:4300) during local development.

Key points

1 . baseHref

Resource URLs need to start with /angular_osf to be properly proxied. baseHref is set to /angular_osf/assets/, ensuring that URLs such as /@vite/client resolve correctly.

2 . Cookie authentication

Although a dedicated login UI is not included, the app now reads the standard auth cookies. If the user is already authenticated, their username appears in the top-right corner. One possible verification flow is as follows:

1. Run osf.io with PRIMARY_WEB_APP="ember_osf_web" (osf.io/website/settings/local.py).
2. Browse to http://localhost:5000 and sign in via ember-osf-web.
3. Switch PRIMARY_WEB_APP to "angular_osf" and restart the web service.
4. Reload http://localhost:5000; angular-osf loads and displays the logged-in user.

[chiku-samugari]

I found that the Cookie Authentication feature proposed in this draft PR has been implemented, in a better approach than mine. I close this PR for that reason.
Many thanks to everyone who took the time to review and consider it.