fix: improve session export and empty chat states

Move session export into the sidebar menu
Add desktop save-and-reveal flow for exported markdown
Show empty-state UI instead of a loading skeleton for empty sessions

packages/ui/src/lib/desktop.ts

@@ -502,6 +502,32 @@ export const openDesktopPath = async (path: string, app?: string | null): Promis
   }
 };
 
+export const saveDesktopMarkdownFile = async (
+  defaultFileName: string,
+  content: string,
+): Promise<string | null> => {
+  if (!isTauriShell() || !isDesktopLocalOriginActive()) {
+    return null;
+  }
+
+  const trimmedFileName = defaultFileName?.trim();
+  if (!trimmedFileName) {
+    return null;
+  }
+
+  try {
+    const tauri = (window as unknown as { __TAURI__?: TauriGlobal }).__TAURI__;
+    const result = await tauri?.core?.invoke?.('desktop_save_markdown_file', {
+      defaultFileName: trimmedFileName,
+      content,
+    });
+    return typeof result === 'string' && result.trim().length > 0 ? result : null;
+  } catch (error) {
+    console.warn('Failed to save markdown file (tauri)', error);
+    return null;
+  }
+};
+
 export const openDesktopProjectInApp = async (
   projectPath: string,
   appId: string,