feat(marketplace): add usage analytics for template authors

Enhanced the Creator Dashboard with comprehensive analytics:

- 4 summary cards: Published, Total Installs, Avg Rating, Total Reviews
- Install trend area chart (30-day daily + cumulative)
- Rating distribution bar chart (1-5 stars)
- Per-agent performance table (installs, rating, reviews, publish date)
- Recent reviews feed with star display

DB layer: fetchCreatorAnalytics() collects per-agent stats, time-bucketed
install trends, rating distribution, and recent reviews from existing
agent_installs, agent_reviews, and agents tables. No migrations needed.

Hook: useCreatorAnalytics() React Query wrapper with 1-min stale time.

Author: vincentgrobler