feat(a11y): make pricing cards keyboard accessible

[google-labs-jules]

This PR addresses an accessibility violation where the pricing plan cards were implemented as non-semantic <div> elements, making them inaccessible to keyboard and screen reader users.

Changes:

• Converted the div cards to semantic buttons using role="button".
• Added tabIndex={0} to ensure they are part of the tab sequence.
• Implemented aria-label using the pattern "Select [Plan Name] plan - $[Price] per month" to provide clear context.
• Added an onKeyDown handler to support Enter and Space key activation, adhering to WAI-ARIA practices for custom buttons.
• Added focus-visible styles to provide a visual indicator for keyboard navigation.

Verification:

• Verified using a custom Playwright script (verify_pricing.py) which confirmed:
  • Elements are reachable via get_by_role("button").
  • Correct aria-labels are present (e.g., "Select Operator plan - $19 per month").
  • tabindex is correct.
• Visually verified via screenshot that the UI remains consistent.
• Ran npm run lint in frontend/ to ensure no new errors were introduced (existing errors in other files persist).

Note:
The linting run showed warnings for unused variables e in the new handlers. These are kept for potential future expansion or standard event signature compliance but could be underscored if strict linting is enforced. The existing errors in AgentVisualizer.tsx and SoundManager.ts are unrelated to this change.

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PR created automatically by Jules for task 1161364210111156752 started by @cjo93

[google-labs-jules]

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