Add CodeMirror 6 for markdown editing

- Install CodeMirror 6 with markdown language support
- Add MarkdownEditor component with:
  - Syntax highlighting for markdown
  - Line numbers
  - Active line highlighting
  - History (undo/redo)
  - One Dark theme
  - Code block language highlighting
- Keep react-markdown for preview mode
- Add CSS for CodeMirror integration

1 files changed, 14 insertions, 0 deletions

diff --git a/renderer/src/styles/globals.css b/renderer/src/styles/globals.css
index 384ebc5..f115734 100644
--- a/renderer/src/styles/globals.css
+++ b/renderer/src/styles/globals.css
@@ -152,6 +152,20 @@ body {
   color: var(--text-primary);
 }
 
+.codemirror-editor {
+  flex: 1;
+  overflow: hidden;
+}
+
+.codemirror-editor .cm-editor {
+  height: 100%;
+}
+
+.codemirror-editor .cm-gutters {
+  background: var(--bg-secondary);
+  border-right: 1px solid var(--border);
+}
+
 .document-content.rendered {
   line-height: 1.7;
 }