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.ts

@@ -111,7 +111,19 @@ export function useCodeMirror(
 
     const view = new EditorView({ state, parent })
     viewRef.current = view
-    return () => { view.destroy(); viewRef.current = null }
+
+    // When CSS zoom changes on the document, CodeMirror's cached measurements
+    // (scaleX/scaleY, line heights, character widths) become stale because
+    // ResizeObserver doesn't fire for ancestor zoom changes. Force a re-measure
+    // so cursor placement stays accurate at any zoom level.
+    const handleZoomChange = () => { view.requestMeasure() }
+    window.addEventListener('laputa-zoom-change', handleZoomChange)
+
+    return () => {
+      window.removeEventListener('laputa-zoom-change', handleZoomChange)
+      view.destroy()
+      viewRef.current = null
+    }
     // Re-create editor when isDark changes (theme is baked into extensions)
     // eslint-disable-next-line react-hooks/exhaustive-deps
   }, [isDark])