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/src/pages/run/RunDetailPage.tsx

@@ -119,11 +119,6 @@ export function RunDetailPage() {
       width: 80,
       render: (s) => <Badge variant="neutral">{s.scenarioStep?.type ?? '–'}</Badge>,
     },
-    {
-      key: 'session',
-      header: t('runs.step_session'),
-      render: (s) => s.scenarioStep?.sessionName ?? '–',
-    },
     {
       key: 'status',
       header: t('runs.step_status'),