Expand top-to-bottom split resize target

[brsbl]

Summary

• make split divider hit regions explicit and symmetric across both orientations
• preserve the one-pixel visual seam and existing resize persistence behavior
• add focused top-to-bottom drag regression coverage

Verification

• pnpm exec turbo run test --filter=@bb/app -- --run src/views/thread-detail/SplitThreadArea.test.tsx (33 passed)
• pnpm exec turbo run typecheck --filter=@bb/app

[brsbl]

Final stack update:

• Rebased onto PR Add automation breadcrumbs and pane full-screen controls #953 at 76d9d32; top head is a6463b8.
• Top-to-bottom split dividers retain the one-pixel visual seam and now expose the same effective 12px drag target as left-to-right splits.
• Focused top-stack pane suite: 46 tests passed, including both resize orientations and the aligned pane-header controls.
• Full combined package matrix passed locally; BB desktop QA confirmed both orientations resize easily.
• No unresolved review threads.

Not merged.