fix: gate command palette entry behind table detection

Previously the command silently no-oped when run on a non-table
selection. Use editorCheckCallback so the entry is grayed out in the
palette unless the selection contains table-like characters, matching
the existing right-click menu behavior.

src/main.ts

@@ -23,8 +23,11 @@ export default class TableBeautifierPlugin extends Plugin {
     this.addCommand({
       id: "convert-table-to-markdown",
       name: "Convert table to Markdown",
-      editorCallback: (editor: Editor) => {
-        convertSelection(editor);
+      editorCheckCallback: (checking: boolean, editor: Editor) => {
+        const selection = editor.getSelection();
+        if (!selection || !looksLikeTable(selection)) return false;
+        if (!checking) convertSelection(editor);
+        return true;
       },
     });