feat(scenarios): add step title, scenario credentials, and environment context in executor

- add nullable title column to scenario steps; exposed in create/edit forms and step table
- add scenario-credential join (many-to-many with CredentialEntity) with CRUD endpoints
- expose environment URLs in code executor helpers via helpers.env and helpers.getEnvUrl()
- resolve and cache environment per run from the login step's environmentName in scheduler
- add section spacing and step table title column to ScenarioDetailPage

client/vite.config.ts

@@ -13,6 +13,7 @@ export default defineConfig({
   server: {
     proxy: {
       '/environments': 'http://localhost:13000',
+      '/credentials': 'http://localhost:13000',
       '/sessions': 'http://localhost:13000',
       '/scenarios': 'http://localhost:13000',
       '/keys': 'http://localhost:13000',