Add path-prefix deployment support for migrating to podsteadr's domain

Regress needs to live at podsteadr.atobitcoin.io/regress/ since / is
already podsteadr. Two coordinated pieces:
- VITE_BASE build arg (frontend asset/script paths, %BASE_URL% in
  index.html for the nostr-provider.js script tag)
- ROUTE_PREFIX runtime env (backend routes registered under the prefix
  via Fastify's plugin-encapsulation, including /api/health)

The nginx location must forward the prefix unstripped (proxy_pass with
no trailing path) — NIP-98 login signs the exact URL it calls, so a
stripped prefix makes the backend reconstruct a different URL than what
was signed and every login fails. Caught this with a real nginx+docker
integration test locally before it could break the live migration, then
fixed a matching bug in auth.ts (it was signing the unprefixed URL while
api.ts fetched the prefixed one). New routePrefix.test.ts proves the
prefix is actually enforced, including a negative case. 40 tests passing.

Also fixes a latent bug: import.meta.env usage had no vite/client type
reference, so it only ever passed typecheck by accident in earlier local
runs — added the standard vite-env.d.ts.

frontend/vite.config.ts

@@ -2,6 +2,7 @@ import { defineConfig } from 'vite';
 import vue from '@vitejs/plugin-vue';
 
 export default defineConfig({
+  base: process.env.VITE_BASE || '/',
   plugins: [vue()],
   server: {
     proxy: {