fix: make create work cards actionable

[codexbounty01cf6f95]

Summary

Refs #31

This PR makes the "Get help from an expert" cards actionable instead of static-looking cards that do not navigate anywhere.

What Changed

• Adds explicit create-work destinations to siteConfig for bounties and tasks.
• Converts the three "Looking for Help" cards into full-card links with visible CTAs.
• Updates the card set to reflect current Gibwork creation modes: Open Source Bounty, Public Task, and Private Task.
• Adds accessible labels, keyboard focus rings, and rel="noopener noreferrer" for external links.
• Fixes stale repeated alt text by using each card title as the image alt text.

Why

Issue #31 reports that the cards look clickable but do nothing. This creates a conversion dead end in the main product section of the landing page.

This keeps the fix narrow: it does not redesign the landing page or duplicate broader mobile/hero refresh PRs already open. It resolves the specific broken interaction and routes users directly into the correct Gibwork creation flows.

Verification

• git diff --check
• tsc --noEmit
• next build
• Local browser smoke test at http://localhost:3000
• Confirmed rendered card links:
  • Create bounty -> https://app.gib.work/bounties/create
  • Create public task -> https://app.gib.work/tasks/create
  • Create private task -> https://app.gib.work/tasks/create

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