Set PUBLIC_API_URL at build time for production

Vite inlines import.meta.env.PUBLIC_* at build time, not runtime.
Without this, the frontend falls back to localhost:8099 which doesn't
exist from the browser. Empty string means same-origin requests,
letting Caddy route /api/* to the backend.

frontend/Dockerfile

@@ -24,6 +24,12 @@ RUN npm ci
 COPY . .
 
 ENV ASTRO_TELEMETRY_DISABLED=1
+
+# PUBLIC_API_URL must be set at build time for Vite to inline it.
+# Empty string = same-origin (Caddy routes /api/* to backend).
+ARG PUBLIC_API_URL=""
+ENV PUBLIC_API_URL=${PUBLIC_API_URL}
+
 RUN npm run build