fix: CodeMirror cursor placement at non-100% zoom levels

Root cause: CSS zoom on document.documentElement caused stale CodeMirror
measurements. Two issues:

1. Race condition: zoom was applied in useEffect (parent), but CodeMirror
   was created in useEffect (child) — child effects run first, so CM
   measured at zoom=1 before zoom was actually applied.

2. No re-measure on zoom change: CSS zoom changes don't trigger
   ResizeObserver on descendant elements, so CodeMirror never updated
   its cached scaleX/scaleY, line heights, or character widths.

Fix:
- Apply zoom synchronously during useState init (before child effects)
- Dispatch 'laputa-zoom-change' event when zoom changes
- Listen for this event in useCodeMirror and call view.requestMeasure()

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/hooks/useCodeMirror.test.ts

@@ -0,0 +1,69 @@
+import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest'
+import { renderHook, act } from '@testing-library/react'
+import { useCodeMirror, type CodeMirrorCallbacks } from './useCodeMirror'
+
+const noop = () => {}
+const noopCallbacks: CodeMirrorCallbacks = {
+  onDocChange: noop,
+  onCursorActivity: noop,
+  onSave: noop,
+  onEscape: () => false,
+}
+
+describe('useCodeMirror', () => {
+  let container: HTMLDivElement
+
+  beforeEach(() => {
+    container = document.createElement('div')
+    document.body.appendChild(container)
+  })
+
+  afterEach(() => {
+    document.body.removeChild(container)
+  })
+
+  it('creates an EditorView in the container', () => {
+    const ref = { current: container }
+    const { result } = renderHook(() =>
+      useCodeMirror(ref, 'hello world', false, noopCallbacks),
+    )
+    expect(result.current.current).not.toBeNull()
+    expect(container.querySelector('.cm-editor')).toBeInTheDocument()
+  })
+
+  it('calls requestMeasure when laputa-zoom-change event fires', () => {
+    const ref = { current: container }
+    const { result } = renderHook(() =>
+      useCodeMirror(ref, 'hello', false, noopCallbacks),
+    )
+    const view = result.current.current!
+    const spy = vi.spyOn(view, 'requestMeasure')
+
+    act(() => {
+      window.dispatchEvent(new Event('laputa-zoom-change'))
+    })
+
+    expect(spy).toHaveBeenCalled()
+    spy.mockRestore()
+  })
+
+  it('stops listening for zoom changes after unmount', () => {
+    const ref = { current: container }
+    const { result, unmount } = renderHook(() =>
+      useCodeMirror(ref, 'hello', false, noopCallbacks),
+    )
+    const view = result.current.current!
+    const spy = vi.spyOn(view, 'requestMeasure')
+
+    unmount()
+
+    act(() => {
+      window.dispatchEvent(new Event('laputa-zoom-change'))
+    })
+
+    // After unmount, the listener should be removed — requestMeasure should NOT be called.
+    // (The view is also destroyed on unmount, so this verifies cleanup.)
+    expect(spy).not.toHaveBeenCalled()
+    spy.mockRestore()
+  })
+})