Fix Bootstrap compatibility and UI issues in user_tokens.html template

- Replace Bootstrap 5 classes and components with custom CSS compatible with base template
- Convert inline onclick handlers to data attributes with event delegation
- Add comprehensive custom CSS for modals, forms, cards, badges, and buttons
- Fix JavaScript template variable issues using data attributes
- Implement modern clipboard API with fallback for older browsers
- Add proper event listeners for token management actions (revoke, extend, delete)
- Template now works with existing custom CSS framework instead of Bootstrap