feat(explore): show helpful message when curated/community tab has no matches but the other does

[cqnykamp]

Summary

Fixes #2771

• When the Curated tab has no matches but the Community tab does, display a context-aware message ("No Curated Matches Found! There are X Community matches.") with a button to switch to the Community tab.
• The reverse is also handled: when the Community (searched) tab has no matches but Curated does, the message directs to the Curated tab.
• Broadens CardList's emptyMessage prop from string to ReactNode to allow richer empty states.

Test plan

• Create a public (non-library) activity and search for its unique name on /explore — with the Curated tab selected, you should see "No Curated Matches Found!" plus the community count and a "Switch to Community tab" button.
• Click the "Switch to Community tab" button and verify the Community tab becomes active.
• Create a library-published activity and search for its unique name on /explore — with the Community tab selected and no community matches, you should see "No Community Matches Found!" plus the curated count and a "Switch to Curated tab" button.
• When both tabs have results (or both are empty), the generic "No Matches Found!" still appears.
• New Cypress e2e test (navigateExplore.cy.ts) covers the curated-empty/community-has-results case.

https://claude.ai/code/session_01URNLszQkbKKoDp7ChRdr88

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