UI/UX improvements for new Plate Tectonics model

[breity]

• Change prompt for prediction text to something like "Explain why you chose these plate and boundary types."
• Reorder prediction tasks for more logical UX (from top to bottom):
  1. Plate type with prompt
  2. Boundary type with prompt
  3. Explanation text with prompt
  4. Home, Play, Reset buttons
• Add keyboard navigation support
• Improve accessibility:
  • 'alt' attributes for all images
  • 'aria-label' for buttons