Add integration tests for #519, #521, #523, and #525

[kingbitnation]

Summary

• Add integration test for activity feed showing correct creator handle for each trade entry #523 — Activity feed shows formatted creator handles (@arivers, @schen_dev) instead of raw creator IDs; integration test covers two trades from different creators.
• Add integration test for portfolio page showing correct entry count in header #521 — Holdings overview header count tracks held positions separately from the marketplace browse list; integration tests cover 3 entries, empty response, and refresh.
• Add integration test for debounced search clearing results when input is emptied #519 — Integration test confirms clearing the debounced search input refetches without a search param and restores the full creator list.
• Add integration test for marketing page rendering correctly with no JavaScript errors on load #525 — Integration test confirms MarketingPage renders all major sections without console errors or warnings.

Test plan

• pnpm exec vitest run src/components/common/__tests__/TransactionHistory.creatorHandle.integration.test.tsx
• pnpm exec vitest run src/pages/__tests__/LandingPage.holdingsCount.integration.test.tsx
• pnpm exec vitest run src/pages/__tests__/LandingPage.debouncedSearchClear.integration.test.tsx
• pnpm exec vitest run src/pages/__tests__/MarketingPage.integration.test.tsx
• pnpm lint
• pnpm build

Closes #519
Closes #521
Closes #523
Closes #525

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