feat(split-tabs): add top-bottom split panes

[Wei-Cheng Hsu (weicheng-hsu)]

Summary

Adds top/bottom split creation from the split tabs toolbar button context menu.

• Keeps the existing toolbar left-click behavior unchanged.
• Adds context menu options for left/right and top/bottom split creation.
• Adds browser test coverage for the toolbar context menu flow.

Design

This PR adds a layout parameter to the toolbar split creation path and passes it through to the split view UI.

The top/bottom option uses SplitTabLayout::kHorizontal, which MultiContentsView uses to lay out panes vertically and resize them by height ratio. Existing default split
creation paths continue to use the current left/right layout.

Test plan

• Built BrowserOS locally with full autoninja.
• Manually verified top/bottom split creation from the toolbar button context menu.
• Manually verified focus switching and divider resizing for top/bottom panes.

Related to: #928

split_top_bottom_test.mp4

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[Wei-Cheng Hsu (weicheng-hsu)]

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