feat: replace CodeMirror 6 with Milkdown 7.21 editor

- Remove 7 @codemirror/* packages, add 7 @milkdown/* packages
- Rewrite useCodeMirror -> useMilkdown hook (ProseMirror-based)
- Rewrite Editor.tsx with Milkdown lifecycle management
- Update EditorToolbar to use Milkdown callCommand API
- Replace CodeMirror CSS styles with Milkdown/ProseMirror styles
- Support: CommonMark + GFM, dark mode, toolbar formatting
- Bump version 0.2.0 -> 0.3.0

package.json

@@ -1,6 +1,6 @@
 {
   "name": "marklite",
-  "version": "0.2.0",
+  "version": "0.3.0",
   "description": "Lightweight Markdown Editor for Windows",
   "main": "./dist/main/index.js",
   "scripts": {
@@ -28,13 +28,13 @@
   "author": "MarkLite",
   "license": "MIT",
   "dependencies": {
-    "@codemirror/commands": "^6.10.3",
-    "@codemirror/lang-markdown": "^6.5.0",
-    "@codemirror/language": "^6.12.3",
-    "@codemirror/search": "^6.7.0",
-    "@codemirror/state": "^6.6.0",
-    "@codemirror/theme-one-dark": "^6.1.3",
-    "@codemirror/view": "^6.43.0",
+    "@milkdown/core": "^7.21.1",
+    "@milkdown/ctx": "^7.21.1",
+    "@milkdown/preset-commonmark": "^7.21.1",
+    "@milkdown/preset-gfm": "^7.21.1",
+    "@milkdown/prose": "^7.21.2",
+    "@milkdown/react": "^7.21.1",
+    "@milkdown/utils": "^7.21.1",
     "dexie": "^4.0.11",
     "nanoid": "^5.1.5",
     "react": "^18.3.1",