feat: replace raw editor textarea with CodeMirror 6

Adds line numbers, current line highlight, and syntax highlighting
(YAML frontmatter keys/values, --- delimiters, markdown headings).
Extracted useCodeMirror hook and frontmatterHighlight extension.
Preserves wikilink autocomplete, Cmd+S save, and Escape dismiss.

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

src/hooks/useCodeMirror.ts

@@ -0,0 +1,119 @@
+import { useRef, useEffect } from 'react'
+import { EditorView, lineNumbers, highlightActiveLine, keymap } from '@codemirror/view'
+import { EditorState } from '@codemirror/state'
+import { defaultKeymap, history, historyKeymap } from '@codemirror/commands'
+import { frontmatterHighlightPlugin, frontmatterHighlightTheme } from '../extensions/frontmatterHighlight'
+
+const FONT_FAMILY = '"Berkeley Mono", "JetBrains Mono", "Fira Mono", ui-monospace, "SFMono-Regular", Menlo, Monaco, Consolas, "Liberation Mono", "Courier New", monospace'
+
+export interface CodeMirrorCallbacks {
+  onDocChange: (doc: string) => void
+  onCursorActivity: (view: EditorView) => void
+  onSave: () => void
+  onEscape: () => boolean
+}
+
+function buildBaseTheme(isDark: boolean) {
+  const bg = isDark ? '#1e1e1e' : '#ffffff'
+  const fg = isDark ? '#d4d4d4' : '#1e1e1e'
+  const gutterBg = isDark ? '#1e1e1e' : '#ffffff'
+  const gutterColor = isDark ? '#555' : '#aaa'
+  const activeLineBg = isDark ? 'rgba(255,255,255,0.04)' : 'rgba(0,100,255,0.06)'
+  const gutterBorder = isDark ? '#333' : '#eee'
+
+  return EditorView.theme({
+    '&': {
+      fontSize: '13px',
+      fontFamily: FONT_FAMILY,
+      backgroundColor: bg,
+      color: fg,
+      flex: '1',
+      minHeight: '0',
+    },
+    '.cm-scroller': {
+      fontFamily: FONT_FAMILY,
+      lineHeight: '1.6',
+      padding: '16px 0',
+      overflow: 'auto',
+    },
+    '.cm-content': {
+      padding: '0 32px 0 16px',
+      caretColor: fg,
+    },
+    '.cm-gutters': {
+      backgroundColor: gutterBg,
+      color: gutterColor,
+      borderRight: `1px solid ${gutterBorder}`,
+      paddingLeft: '16px',
+    },
+    '.cm-lineNumbers .cm-gutterElement': {
+      paddingRight: '12px',
+      minWidth: '28px',
+      textAlign: 'right',
+    },
+    '.cm-activeLine': {
+      backgroundColor: activeLineBg,
+    },
+    '.cm-activeLineGutter': {
+      backgroundColor: activeLineBg,
+    },
+    '&.cm-focused': { outline: 'none' },
+    '.cm-line': { padding: '0' },
+  }, { dark: isDark })
+}
+
+function buildSaveKeymap(callbacks: { current: CodeMirrorCallbacks }) {
+  return keymap.of([{
+    key: 'Mod-s',
+    run: () => { callbacks.current.onSave(); return true },
+  }, {
+    key: 'Escape',
+    run: () => callbacks.current.onEscape(),
+  }])
+}
+
+export function useCodeMirror(
+  containerRef: React.RefObject<HTMLDivElement | null>,
+  content: string,
+  isDark: boolean,
+  callbacks: CodeMirrorCallbacks,
+) {
+  const viewRef = useRef<EditorView | null>(null)
+  const callbacksRef = useRef(callbacks)
+  callbacksRef.current = callbacks
+
+  useEffect(() => {
+    const parent = containerRef.current
+    if (!parent) return
+
+    const state = EditorState.create({
+      doc: content,
+      extensions: [
+        lineNumbers(),
+        highlightActiveLine(),
+        history(),
+        keymap.of([...defaultKeymap, ...historyKeymap]),
+        buildSaveKeymap(callbacksRef),
+        buildBaseTheme(isDark),
+        frontmatterHighlightTheme(isDark),
+        frontmatterHighlightPlugin,
+        EditorView.updateListener.of((update) => {
+          if (update.docChanged) {
+            callbacksRef.current.onDocChange(update.state.doc.toString())
+          }
+          if (update.selectionSet || update.docChanged) {
+            callbacksRef.current.onCursorActivity(update.view)
+          }
+        }),
+      ],
+    })
+
+    const view = new EditorView({ state, parent })
+    viewRef.current = view
+    return () => { 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])
+
+  return viewRef
+}