chore: add .env.production and simplify Dockerfile

Bake VITE_DIRECTUS_URL into production build via .env.production
so no Coolify build-arg configuration is needed.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.env.production

@@ -0,0 +1 @@
+VITE_DIRECTUS_URL=https://db.hejyou.com

Dockerfile

@@ -5,10 +5,6 @@ COPY package*.json ./
 RUN npm ci
 
 COPY . .
-
-ARG VITE_DIRECTUS_URL
-ENV VITE_DIRECTUS_URL=$VITE_DIRECTUS_URL
-
 RUN npm run build
 
 FROM nginx:alpine