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/scenario/ScenarioDetailPage.tsx

@@ -117,7 +117,7 @@ export function ScenarioDetailPage() {
     {
       key: 'title',
       header: t('scenarios.step_title'),
-      render: (s) => s.title ?? <span style={{ color: 'var(--color-text-muted)', fontStyle: 'italic' }}>{s.sessionName}</span>,
+      render: (s) => s.title ?? <span style={{ color: 'var(--color-text-muted)', fontStyle: 'italic' }}>{'—'}</span>,
     },
     {
       key: 'updated',