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.

Dockerfile

@@ -1,10 +1,11 @@
 # ---- frontend ----
 FROM node:22-bookworm-slim AS frontend-build
+ARG VITE_BASE=/
 WORKDIR /build/frontend
 COPY frontend/package*.json ./
 RUN npm ci
 COPY frontend/ ./
-RUN npm run build
+RUN VITE_BASE=$VITE_BASE npm run build
 
 # ---- server ----
 FROM node:22-bookworm-slim AS server-build
@@ -16,6 +17,7 @@ RUN npm run build && npm prune --omit=dev
 
 # ---- runtime ----
 FROM node:22-bookworm-slim
+ARG ROUTE_PREFIX=
 RUN apt-get update \
     && apt-get install -y --no-install-recommends curl \
     && rm -rf /var/lib/apt/lists/*
@@ -29,8 +31,9 @@ USER node
 ENV NODE_ENV=production \
     PORT=8199 \
     DATA_DIR=/data \
-    STATIC_DIR=/app/public
+    STATIC_DIR=/app/public \
+    ROUTE_PREFIX=${ROUTE_PREFIX}
 EXPOSE 8199
 HEALTHCHECK --interval=30s --timeout=5s --retries=3 \
-    CMD curl -fsS http://localhost:8199/api/health || exit 1
+    CMD curl -fsS http://localhost:8199${ROUTE_PREFIX}/api/health || exit 1
 CMD ["node", "dist/index.js"]