fix(layout): enlarge tiling scrollbar hit area

[OliverBennettdev]

Description

Increases the horizontal scrollbar hit area for the tiling layout strip from 5px to 10px while keeping the painted thumb approximately 6px tall.

The tiling strip previously inherited the global 5px scrollbar size, making it difficult to target. Missing the scrollbar could also place the pointer over nearby panel resize handles.

This change:

• scopes the larger scrollbar hit area to .tiling-layout-strip
• preserves the existing 5px scrollbar size elsewhere
• uses transparent thumb borders to retain a compact visual appearance
• adds a CSSOM regression test covering both the scoped and global scrollbar dimensions

Issues Resolved

Fixes #255

Check List

• New functionality includes testing.
• New functionality has been documented in the README if applicable. (Not applicable; this change introduces no new configuration or user-facing API.)