fix: use absolute asset paths and prevent route-level runtime navigation

Change vite base from './' to '/' so asset paths are absolute. With
relative paths, direct navigation to multi-segment URLs like
/runtime/app/instance resolved assets to /runtime/assets/ which 404'd.

Also fix sidebar navigation: clicking a route while on the runtime tab
no longer navigates to /runtime/{appId}/{routeId} (which the runtime
page interprets as an instanceId). It stays at /runtime/{appId}.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ui/vite.config.ts

@@ -27,7 +27,7 @@ export default defineConfig({
   optimizeDeps: {
     include: ['swagger-ui-dist/swagger-ui-bundle'],
   },
-  base: './',
+  base: '/',
   build: {
     outDir: 'dist',
   },