feat: configurable UI base path via BASE_PATH build arg (#58)

Serves the UI under /ui instead of /. This pairs with the argocd route simplification (argocd-apps#201) where /ui → UI service and everything else → API.

- Vite: `base` set from `BASE_PATH` env var at build time
- React Router: `basename` set from injected `__BASE_PATH__`
- Nginx: location block uses `${BASE_PATH}`, substituted by sed at build
- Dockerfile: `ARG BASE_PATH=/` (default preserves existing behavior)
- Woodpecker: passes `BASE_PATH=/ui` to docker-web build

Tested: assets serve at `/ui/assets/...`, SPA routing works at `/ui/remotes`, etc.
Reviewed-on: #58
Co-authored-by: Ben Vincent <ben@unkin.net>
Co-committed-by: Ben Vincent <ben@unkin.net>

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;"]