feat(shop): add loading skeleton and empty state to products grid (closes #105)

[Ranjeet2063]

Summary of Changes

Resolves issue #105 by adding loading and empty states to the shop products grid in app/shop/page.jsx.

Key Changes

1. Loading State with Skeleton:
   • Added loading state (defaults to true) during initial listProducts fetch.
   • Imported and rendered ProductGridSkeleton from components/Skeleton.tsx while loading is true.
2. Explicit Empty State:
   • When loading resolves and products.length === 0 without error, displays an explicit "No products yet" empty message.
3. Preserved Error State:
   • Maintains the existing error alert if listProducts rejects.
4. Comprehensive Unit Tests:
   • Added tests/app/shop/page.test.tsx testing:
     1. ProductGridSkeleton is rendered while pending.
     2. "No products yet" empty message renders on empty array.
     3. Error alert renders on rejection.
     4. Product cards grid renders when products are loaded.

Verification (2x Verified)

• npx vitest run tests/app/shop/page.test.tsx (4/4 tests pass)
• npm run type-check (0 errors)
• npm run lint (0 errors)
• npx prettier --check (100% formatted)

Closes #105

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