Add Ghostty-style drag-to-resize for terminal split panes

[nwparker]

Summary

• Wider hit area: Dividers use a CSS ::after pseudo-element for the visible line, while the element itself is a wider transparent grab zone (~10px total). This prevents the old bug where background shorthand resets wiped background-clip.
• Persist split ratios: Serialization now captures flex-grow ratios into the layout snapshot (ratio field on split nodes), and replayTerminalLayout restores them — resized panes survive tab switches.
• Double-click to equalize: Double-clicking a divider resets both sibling panes to 50/50, matching Ghostty behavior.
• Layout change on drag-end: onLayoutChanged fires after a real drag so ratio persistence is triggered automatically.

Test plan

• Split a terminal (Cmd+D / Cmd+Shift+D), verify blue divider line is visible
• Hover divider — should highlight even in the transparent padding zone
• Drag divider to resize panes, verify smooth real-time resize
• Switch tabs and back — verify resized ratios are preserved
• Double-click divider — panes should snap back to equal size
• Nested splits (3+ panes) — verify dividers and resize work at all levels

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