fix: Disable pointer events on closed chat panel (#11561)
## Summary
- Fix mobile header buttons (search, Ask AI, hamburger menu) being
unclickable
The chat panel is a fixed-position element that slides off-screen when
closed via `translate-x-full`. However, it still captured pointer
events, blocking clicks on header buttons positioned underneath.
Adding `pointer-events-none` when closed and `pointer-events-auto` when
open resolves this.
## Test
1. Resize browser to mobile width (< 768px)
2. Verify search, Ask AI, and hamburger menu buttons are clickable
