Add dashboard shell with sidebar nav and version header.

Replace the health-check stub UI with a left menu and content panel so section navigation and frontend/backend versions are visible in the shell.

webui/vite.config.ts

@@ -1,8 +1,16 @@
+import { readFileSync } from 'node:fs'
 import { defineConfig } from 'vitest/config'
 import react from '@vitejs/plugin-react'
 
+const packageJson = JSON.parse(
+  readFileSync(new URL('./package.json', import.meta.url), 'utf-8'),
+) as { version: string }
+
 export default defineConfig({
   plugins: [react()],
+  define: {
+    __FRONTEND_VERSION__: JSON.stringify(packageJson.version),
+  },
   server: {
     proxy: {
       '/health': 'http://localhost:8080',