Fix mobile tab scrolling and transition invalidation

[hamzamerzic]

Summary

• let horizontal touch swipes scroll an overflowing mobile tab strip while reserving the visible grip for touch reordering
• keep mouse whole-body dragging and vertical mobile body dragging unchanged
• keep the generic data-drag-key unique to the tab button and identify the nested grip through its dedicated data-touch-drag-handle contract
• invalidate both entry and exit transitions when the captured layout, topology, or destination drifts
• add real-browser coverage for overflow scrolling and handle-based reorder

Validation

Amended head f9fcfc358c229ac78c40f898cb3aba06d8829ac3:

• focused frontend unit/contract suite: 180 passed
• disposable Playwright: 5 passed (authentication; handle reorder; body overflow scroll without reorder; builder single-leaf selector; middle-click close selector)
• the latter two cases reproduce and verify the selector collision found by the first hosted run

Earlier full validation before the final selector-contract amendment:

• full frontend suite: 1,751 tests and 47 contract tests passed
• production frontend build: passed

Based on the post-#111 main commit efe4a1a92ccfe3bf40109115f953ef84130077d3.