Add keyboard navigation for category budget cards

[maxmini0214]

Summary

The budget category cards in the Structure tab should be fully navigable via keyboard.

Current behavior

Category cards are clickable but not fully focusable via Tab key.

Expected behavior

• Tab cycles through categories
• Enter/Space opens edit mode
• Escape cancels editing

Why

Accessibility matters. Keyboard-only users should be first-class citizens.

Difficulty

Good first issue — mostly tabIndex, onKeyDown, and ARIA attributes