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, 6 insertions, 0 deletions

diff --git a/package.json b/package.json
index 7bc98e3..9583737 100644
--- a/package.json
+++ b/package.json
@@ -44,6 +44,12 @@
   },
   "dependencies": {
     "@anthropic-ai/claude-agent-sdk": "^0.2.63",
+    "@codemirror/commands": "^6.10.2",
+    "@codemirror/lang-markdown": "^6.5.0",
+    "@codemirror/language-data": "^6.5.2",
+    "@codemirror/state": "^6.5.4",
+    "@codemirror/theme-one-dark": "^6.1.3",
+    "@codemirror/view": "^6.39.15",
     "better-sqlite3": "12.2.0",
     "react": "^19.1.1",
     "react-dom": "^19.1.1",