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/vite.config.ts

@@ -1,7 +1,10 @@
 import { defineConfig } from 'vite'
 import react from '@vitejs/plugin-react'
 
+const basePath = process.env.BASE_PATH || '/'
+
 export default defineConfig({
+  base: basePath,
   plugins: [react()],
   server: {
     proxy: {
@@ -11,4 +14,7 @@ export default defineConfig({
       '/metrics': 'http://localhost:8000',
     },
   },
+  define: {
+    '__BASE_PATH__': JSON.stringify(basePath),
+  },
 })