test: cover marketplace filters and grid states

[BowenMilner]

Closes #592.

Summary:

• add Vitest/RTL coverage for MarketplaceFilters controls, accessible range labels, filter updates, and reset behavior
• add MarketplaceGrid tests for populated listings, trade links, not-for-sale state, and empty state copy
• cover MarketplaceGridSkeleton loading state with the requested card count

Validation:

• npm test -- tests/components/MarketplaceFilters.test.tsx tests/components/MarketplaceGrid.test.tsx
• npx eslint src/components/MarketplaceFilter/MarketplaceFilters.tsx tests/components/MarketplaceFilters.test.tsx tests/components/MarketplaceGrid.test.tsx src/components/MarketplaceGrid.tsx src/components/MarketplaceGridSkeleton.tsx
• git diff --check

Note: I kept validation focused to the touched marketplace files because the full repo lint currently fails on unrelated existing source issues.

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