Replaces the hover+pin+timeout implementation with Mantine Popover's
built-in uncontrolled behavior (click to open, click outside/escape to
close). Removes all component state, mouse handlers, timeout management,
and the cross-popover close machinery from the parent.
- 56 lines removed, 13 added across 3 files
- Border highlight now via CSS aria-expanded selector
- Resolves the messy hover navigation issue (#2637)