fix: bypass CodeMirror posAtCoords for accurate cursor at non-100% CSS zoom

CSS zoom on document.documentElement causes a coordinate space mismatch
between mouse event clientX/Y (viewport space) and Range.getClientRects()
(CSS space), breaking CodeMirror's click-to-position mapping. The previous
requestMeasure() fix only recalibrated cached geometry, not this mismatch.

New approach: zoomCursorFix extension patches posAtCoords/posAndSideAtCoords
on the EditorView instance to use document.caretRangeFromPoint() — the
browser's native, zoom-aware API — with coord-adjustment fallback.

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

src/hooks/useCodeMirror.ts

@@ -3,6 +3,7 @@ import { EditorView, lineNumbers, highlightActiveLine, keymap } from '@codemirro
 import { EditorState } from '@codemirror/state'
 import { defaultKeymap, history, historyKeymap } from '@codemirror/commands'
 import { frontmatterHighlightPlugin, frontmatterHighlightTheme } from '../extensions/frontmatterHighlight'
+import { zoomCursorFix } from '../extensions/zoomCursorFix'
 
 const FONT_FAMILY = '"Berkeley Mono", "JetBrains Mono", "Fira Mono", ui-monospace, "SFMono-Regular", Menlo, Monaco, Consolas, "Liberation Mono", "Courier New", monospace'
 
@@ -98,6 +99,7 @@ export function useCodeMirror(
         buildBaseTheme(isDark),
         frontmatterHighlightTheme(isDark),
         frontmatterHighlightPlugin,
+        zoomCursorFix(),
         EditorView.updateListener.of((update) => {
           if (update.docChanged) {
             callbacksRef.current.onDocChange(update.state.doc.toString())