feat(gallery): add public templates gallery frontend (Tasks 4 & 5)

Add types, API client, page component, card components, detail modal,
and /templates route for the public templates gallery. Uses raw fetch()
for unauthenticated access, glass-card design system, and URL-synced
filters with debounced search.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/api/index.ts

@@ -28,3 +28,4 @@ export { aiSessionsApi } from './aiSessions'
 export { flowProposalsApi } from './flowProposals'
 export { flowpilotAnalyticsApi } from './flowpilotAnalytics'
 export { notificationsApi } from './notifications'
+export { publicTemplatesApi } from './publicTemplates'