feat(renderer): useScrollSync hook (60fps throttling, ratio calc)

src/renderer/hooks/use-scroll-sync.ts

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+import { useCallback, useRef } from 'react';
+
+interface Options {
+  onEditorScroll?: (ratio: number) => void;
+  onPreviewScroll?: (ratio: number) => void;
+}
+
+export function useScrollSync(opts: Options) {
+  const FRAME_MS = 1000 / 60;
+  const lastTick = useRef(-FRAME_MS);
+
+  const handleEditorScroll = useCallback((evt: React.UIEvent<HTMLElement>) => {
+    const target = evt.currentTarget;
+    const ratio = target.scrollTop / Math.max(target.scrollHeight - target.clientHeight, 1);
+    const now = performance.now();
+    if (now - lastTick.current < FRAME_MS) return;
+    lastTick.current = now;
+    opts.onEditorScroll?.(ratio);
+  }, [opts]);
+
+  const handlePreviewScroll = useCallback((evt: React.UIEvent<HTMLElement>) => {
+    const target = evt.currentTarget;
+    const ratio = target.scrollTop / Math.max(target.scrollHeight - target.clientHeight, 1);
+    opts.onPreviewScroll?.(ratio);
+  }, [opts]);
+
+  return { handleEditorScroll, handlePreviewScroll };
+}

tests/unit/hooks/use-scroll-sync.test.ts

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+import { describe, it, expect, beforeEach, vi } from 'vitest';
+import { renderHook, act } from '@testing-library/react';
+import { useScrollSync } from '@/hooks/use-scroll-sync';
+
+describe('useScrollSync', () => {
+  beforeEach(() => {
+    vi.useFakeTimers();
+    // Mock performance.now() to return 0 and not auto-advance
+    let time = 0;
+    vi.spyOn(performance, 'now').mockImplementation(() => time);
+  });
+
+  it('throttles editor scroll events to 60fps', () => {
+    const onScroll = vi.fn();
+    const { result } = renderHook(() => useScrollSync({ onEditorScroll: onScroll }));
+    const mockEvt = { currentTarget: { scrollTop: 100, scrollHeight: 1000, clientHeight: 200 } } as any;
+    // All calls within the same act() - performance.now() stays at 0
+    // First call passes, subsequent calls within FRAME_MS are throttled
+    act(() => {
+      result.current.handleEditorScroll(mockEvt);
+      result.current.handleEditorScroll(mockEvt);
+      result.current.handleEditorScroll(mockEvt);
+    });
+    expect(onScroll).toHaveBeenCalledTimes(1);
+  });
+});