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/nginx.conf

@@ -5,33 +5,7 @@ server {
     root /usr/share/nginx/html;
     index index.html;
 
-    location /api/ {
-        proxy_pass http://artifactapi:8000;
-        proxy_set_header Host $host;
-        proxy_set_header X-Real-IP $remote_addr;
-        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
-        proxy_set_header X-Forwarded-Proto $scheme;
-        proxy_buffering off;
-    }
-
-    location /v2/ {
-        proxy_pass http://artifactapi:8000;
-        proxy_set_header Host $host;
-        proxy_set_header X-Real-IP $remote_addr;
-        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
-        proxy_set_header X-Forwarded-Proto $scheme;
-        proxy_buffering off;
-    }
-
-    location /health {
-        proxy_pass http://artifactapi:8000;
-    }
-
-    location /metrics {
-        proxy_pass http://artifactapi:8000;
-    }
-
-    location / {
+    location ${BASE_PATH} {
         try_files $uri $uri/ /index.html;
     }
 }