Keep Settings modal drag accessible

[PathGao]

Fixes #311

Moves the pointer listener from the non-interactive header to the existing dialog element, while starting a drag only when the event originated from the header’s non-interactive surface. The close button remains excluded. This preserves mouse dragging without giving a header that contains a button an invalid interactive role.

Adds a regression test for the event ownership and scope.

This is stacked after #310; merge the predecessor chain first.

Validated with npm run check (0 errors, 0 warnings), npm test (136 passing), npm run build, and cargo test (21 passing).

[PathGao]

@alecdotdev Follow-up to #310: the upgraded checker exposed one Settings drag-header accessibility warning. This keeps the existing pointer drag behavior and restores a clean check with a focused regression test. Please review after #310 and its predecessors.