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/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),
+  },
 })