Enhance branch selector field

What does this MR do and why?

Implements responsive sizing for branch/ref selectors to improve usability when working with long branch names. This addresses the issue where the ref selector field was too small and didn't utilize available screen space.

Changelog: changed

Changes include:

• Adding fluid-width to both ref selectors (source and target branch)
• Setting responsive maxWidth values according to viewport breakpoints:
  • Desktop (≥768px): gl-max-w-xs (20rem/320px)
  • Tablet (576px-767px): gl-max-w-4/10 (40%)
  • Mobile (<576px): gl-max-w-full (100%)
• Implementing responsive layout behavior so breadcrumbs stack below the ref selector at narrow viewports

Related to

Closes #438154

How to test

1. Navigate to a repository's commits page or file tree view
2. Test on different viewport sizes:
   • Desktop (1920px+): Ref selector should be constrained to ~320px width
   • Tablet (768px-1024px): Ref selector should take 40% of available width
   • Mobile (<576px): Ref selector should take 100% of available width
3. Verify breadcrumbs wrap below the ref selector on narrow viewports
4. Test with long branch names (e.g., features/developer/version/feature-name) to confirm improved readability

Screenshots or screen recordings

Viewport	Before	After
Desktop (1920px)	)	)
Tablet (768px)	)	)
Mobile (375px)	)	)

Checklist

• Tests added/updated
• Documentation updated (if needed)
• Accessibility verified
• No breaking changes ## Deployment notes No special deployment considerations. This is a UI improvement with no backend changes.

Review checklist

• Code review
• Design review (if applicable)
• QA testing