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.
vite.config.ts hardcoded the /api, /admin, /healthz proxy targets to
http://localhost:3000 — the server's default port — so there was no
way to run dev:client against a dev:server started on a different
port (PORT=<n> pnpm dev:server) without editing the config file.
Read the port from WUTZ_SERVER_PORT (same default of 3000 as the
server's own PORT env var) and build the proxy target once.
