refactor(scenario): remove step type and simplify scheduler to exec-only

- remove StepType, type column, and type field from step entity and DTOs
- remove executeLoginStep, executeSignStep, executeExecStep from scheduler
- inline exec logic directly in executeStepRun; all steps run execCode
- remove AuthService, EnvironmentService, runEnvironments from scheduler
- remove type selector from CreateStepPage and EditStepPage
- remove type badge column from ScenarioDetailPage
- fix useEffect dependency arrays in RunDetailPage (FINAL, id, runId, polling)
- fix duplicate /snippets proxy key in vite.config.ts
- remove unused escapeHtml export from hljs.ts

client/vite.config.ts

@@ -13,7 +13,8 @@ export default defineConfig({
   server: {
     proxy: {
       '/environments': 'http://localhost:13000',
-      '/credentials': 'http://localhost:13000',      '/snippets': 'http://localhost:13000',      '/snippets': 'http://localhost:13000',
+      '/credentials': 'http://localhost:13000',
+      '/snippets': 'http://localhost:13000',
       '/sessions': 'http://localhost:13000',
       '/scenarios': 'http://localhost:13000',
       '/keys': 'http://localhost:13000',