Add role-based input guard for contribution submissions

[JoaquinBN]

Summary

Improved the submission form UX by enforcing role-based access to input fields. Users can now freely switch between Validator and Builder tabs, but the input field is disabled if they lack the required role for the selected category.

Changes

• Added currentCategoryDisabled derived state to check if selected category role is missing
• Made category tabs always clickable (users can freely switch)
• Disabled input field and dropdown when selected category role is missing
• Moved role requirement message below the selector with category-specific text

Test Plan

1. Log in as user without validator or builder role - can switch tabs but inputs are disabled
2. Log in as validator only - validator tab works, builder tab shows message
3. Log in with both roles - all inputs enabled, no messages shown