feat(snippets): add snippets entity, crud, and auto-browser-per-run

- add Snippet entity with name, description, code; full CRUD backend
- add snippets pages (list, create, edit, detail) and nav entry
- add runSnippet helper in code-executor using new Function with args array
- add result param to execute() so validateCode can access exec output
- remove sessionName from steps; each run now spawns its own fresh browser
- fix waitForURL race by polling localStorage for token instead

client/vite.config.ts

@@ -13,7 +13,7 @@ export default defineConfig({
   server: {
     proxy: {
       '/environments': 'http://localhost:13000',
-      '/credentials': 'http://localhost:13000',
+      '/credentials': 'http://localhost:13000',      '/snippets': 'http://localhost:13000',      '/snippets': 'http://localhost:13000',
       '/sessions': 'http://localhost:13000',
       '/scenarios': 'http://localhost:13000',
       '/keys': 'http://localhost:13000',