Replace hardcoded toolbar breakpoints with KListWithOverflow

Use KDS KListWithOverflow component to dynamically collapse toolbar
items into a "More" dropdown based on available space instead of
fixed pixel breakpoints.

- Integrate KListWithOverflow for automatic overflow detection
- Add overflow hidden CSS to prevent flicker during resize
- Simplify toolbar logic by removing manual breakpoint calculations

Fixes learningequality#5258

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: 2 people