feat: add skeleton loading states to Admin Analytics and Rewards pages (Closes #294)

[waterWang]

Summary

Adds layout-matching skeleton loading states to the Admin Analytics and Rewards pages, replacing the generic LoadingState text message.

Changes

• components/admin/analytics-skeleton.tsx — New skeleton component that mirrors the Analytics page layout: stat cards row, membership growth chart (bar-shaped), and a two-column distribution grid
• components/admin/rewards-skeleton.tsx — New skeleton component that mirrors the Rewards page layout: page header, deferred-features notice, member count header, and member reward cards
• app/[communitySlug]/admin/analytics/page.tsx — Import and render AnalyticsSkeleton during loading state instead of generic LoadingState
• app/[communitySlug]/admin/rewards/page.tsx — Import and render RewardsSkeleton during loading state instead of generic LoadingState
• test/skeleton.test.ts — Added tests for both new skeleton components
• test/tsconfig.json — Added new skeleton files to the test project

Testing

✔ skeleton uses theme tokens and stays hidden from assistive technology
✔ membership skeleton reserves the loaded layout and exposes one loading status
✔ analytics skeleton reserves the analytics layout and exposes one loading status
✔ rewards skeleton reserves the rewards layout and exposes one loading status
ℹ tests 4
ℹ pass 4
ℹ fail 0

Closes #294