fix(core): disable toolbar action buttons in preview mode

- _updateModeButtons now disables edit-action buttons in preview mode
- Mode-switch buttons (edit/split/preview/fullscreen) and undo/redo remain active
- CSS: .me-btn:disabled with opacity 0.35, cursor not-allowed, pointer-events none
- Constructor calls _updateModeButtons on init for correct initial state

src/core.js

@@ -137,6 +137,7 @@ class MarkdownEditor {
     setThemeVariables(resolvedThemeConfig, this.el);
 
     this._buildToolbar();
+    this._updateModeButtons();  // 初始化按钮状态
     this._bindEvents();
 
     // 初始内容
@@ -1060,8 +1061,31 @@ class MarkdownEditor {
   getMode() { return this._mode; }
 
   _updateModeButtons() {
-    this.toolbarEl.querySelectorAll('.me-btn[data-mode]').forEach((b) => {
-      b.classList.toggle('me-active', b.dataset.mode === this._mode);
+    const isPreview = this._mode === 'preview';
+    const isEdit = this._mode === 'edit';
+
+    this.toolbarEl.querySelectorAll('.me-btn').forEach((b) => {
+      const mode = b.dataset.mode;
+      const action = b.dataset.action;
+
+      // 模式切换按钮始终可用，高亮当前模式
+      if (mode) {
+        b.classList.toggle('me-active', mode === this._mode);
+        b.disabled = false;
+        return;
+      }
+
+      // preview 模式：禁用编辑操作按钮
+      if (isPreview) {
+        // undo/redo 在 preview 下也可用
+        if (action === 'undo' || action === 'redo' || action === 'fullscreen') {
+          b.disabled = false;
+        } else {
+          b.disabled = true;
+        }
+      } else {
+        b.disabled = false;
+      }
     });
   }