chore: improve production deployment flexibility

- Switch frontend from nginx to caddy for consistency with Coolify
- Make VITE_API_URL optional, auto-derive from window.location.origin/api
- Simplifies deployment by not requiring build-time API URL

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/Dockerfile.prod

@@ -15,20 +15,18 @@ ENV VITE_API_URL=${VITE_API_URL}
 RUN npm run build
 
 # Production stage
-FROM nginx:alpine
+FROM caddy:alpine
 
-COPY --from=builder /app/dist /usr/share/nginx/html
+# Copy built assets
+COPY --from=builder /app/dist /srv
 
-# Handle client-side routing
-RUN echo 'server { \
-    listen 8080; \
-    root /usr/share/nginx/html; \
-    index index.html; \
-    location / { \
-        try_files $uri $uri/ /index.html; \
-    } \
-}' > /etc/nginx/conf.d/default.conf
+# Caddyfile for SPA routing
+RUN echo ':8080 { \
+    root * /srv \
+    file_server \
+    try_files {path} /index.html \
+}' > /etc/caddy/Caddyfile
 
 EXPOSE 8080
 
-CMD ["nginx", "-g", "daemon off;"]
+CMD ["caddy", "run", "--config", "/etc/caddy/Caddyfile"]