[leanderva]

What

The sidebar can be resized by dragging its edge, as asked by the team.

• Width from 220 to 560 px, kept in the browser. It gives way on small windows so the work area keeps 360 px.
• Keyboard: arrow keys move the edge by 16 px, Home and End go to the limits, double click resets to 300 px.
• The handle has the separator role and shows its value to screen readers. It is hidden on narrow screens.

How it was tested

• bun run test:ci: 243 tests pass, 4 of them new for the width limits.
• Browser: dragging to 460 gave 460, dragging to 50 stopped at 220, and the width came back after a reload.

Refs: TEC-3584