The macOS desktop build could lose its draggable gutter on the right side of
the tab bar once multiple session tabs consumed the native overlay title area.
This adds a Tauri window-drag fallback for clicks on the tab strip's empty
space while preserving tab and control interactions, and locks that behavior
with focused regression tests.
Constraint: Must preserve tab clicks, close buttons, and overflow controls while restoring drag behavior
Rejected: Rework the title bar layout around a dedicated spacer | larger UI diff for a narrow hit-testing bug
Confidence: high
Scope-risk: narrow
Reversibility: clean
Directive: Keep drag fallback limited to true empty gutter hits; do not trigger it from interactive tab descendants
Tested: bun run test src/components/layout/TabBar.test.tsx; bun run lint; bun run build; manual macOS app verification
Not-tested: Windows runtime drag behavior was not manually exercised after this change
Related: #92
