Add Dockerfile, nginx config, and Gitea Actions CI workflow

Dockerfile

@@ -0,0 +1,20 @@
+# Stage 1: Build the React app
+FROM node:22-alpine AS builder
+
+WORKDIR /app
+
+COPY package*.json ./
+RUN npm ci
+
+COPY . .
+RUN npm run build
+
+# Stage 2: Serve with nginx
+FROM nginx:alpine
+
+COPY --from=builder /app/dist /usr/share/nginx/html
+COPY --from=builder /app/docker/nginx.conf /etc/nginx/conf.d/default.conf
+
+EXPOSE 80 443
+
+CMD ["nginx", "-g", "daemon off;"]