feat: configurable UI base path via BASE_PATH build arg

Allows serving the UI under a subpath (e.g. /ui) instead of root.

- Vite config reads BASE_PATH env var, sets it as the asset base
  and injects __BASE_PATH__ for runtime use
- React Router uses basename from __BASE_PATH__ for client-side routing
- Nginx config uses ${BASE_PATH} placeholder, replaced at build time
- Dockerfile accepts BASE_PATH build arg (default: /)
- Woodpecker docker pipeline passes BASE_PATH=/ui

ui/Dockerfile.ui

@@ -6,13 +6,20 @@ COPY package.json package-lock.json* ./
 RUN npm ci
 
 COPY . .
+
+ARG BASE_PATH=/
+ENV BASE_PATH=${BASE_PATH}
 RUN npm run build
 
 FROM nginx:alpine
 
+ARG BASE_PATH=/
+
 COPY --from=builder /app/dist /usr/share/nginx/html
 COPY nginx.conf /etc/nginx/conf.d/default.conf
 
+RUN sed -i "s|\${BASE_PATH}|${BASE_PATH}|g" /etc/nginx/conf.d/default.conf
+
 EXPOSE 80
 
 CMD ["nginx", "-g", "daemon off;"]