feat(wizard): restructure AI engine selection flow

Restructure the wizard's Step 2 (AI Engine) into a two-layer choice:
- Orchestrated Agent apps (Dify, n8n, Coze, etc.)
- Direct LLM usage (Space service or custom provider)

For direct LLM mode, users choose between LangBot Space (OAuth)
or adding their own model provider via a dedicated ProviderForm page.

Support intra-step back navigation across all layers.
Add Vite dev server proxy config for API requests.
Add i18n translations for all 8 locales.

web/vite.config.ts

@@ -11,6 +11,12 @@ export default defineConfig({
   },
   server: {
     port: 3000,
+    proxy: {
+      '/api': {
+        target: 'http://127.0.0.1:5300',
+        changeOrigin: true,
+      },
+    },
   },
   build: {
     outDir: 'dist',