feat(ui): split / side-by-side page view

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Summary

• Adds resizable side-by-side page view with independent left/right pane navigation
• Entry points: right-click "Open in Split View" (tree + wiki-links), Mod+\ keyboard shortcut, and history "Compare with current"
• Persists split state in sessionStorage; disabled on viewports <768px with user notice

Implementation

Uses existing kiwi-resize-handle drag pattern for the divider. Core pieces:

• SplitViewProvider context + splitView.ts state helpers
• SplitPageLayout / SplitPageView rendering dual KiwiPage / KiwiEditor panes
• Backend + frontend default keybinding: toggle_split_view: mod+\

Test plan

• go test ./internal/keybindings/... — PASS
• npm test (ui) — 196/196 PASS (includes 7 splitView regression tests)
• Right-click tree item → Open in Split View
• Right-click wiki-link → Open in Split View
• Ctrl+\ / Cmd+\ toggles split view
• Drag divider resizes panes (25–75% clamp)
• Close button on secondary pane
• History → Compare with current opens version split
• Mobile viewport shows blocked message

Upstream: kiwifs#426

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