Fixes the reported /admin MIME-type / NS_ERROR_CORRUPTED_CONTENT breakage on the
Vite dev server. The proxy matched by string prefix, so '/admin' also caught
'/admin' and '/admin.html' (the page requests), forwarding them to the backend
instead of letting Vite serve its own dev-mode HTML. In dev the backend only has
a stale production build (or none) to answer with, so the page loaded referencing
hashed prod asset paths that don't exist in Vite's dev module graph — Vite's dev
server then served its SPA-fallback HTML for those asset requests instead of JS.
Narrowed the proxy to the three backend-owned endpoints under /admin
(api/login/logout) so the bare page routes go through Vite's own dev serving.
b07c7c3602