fix(ui): keep the app root pinned when the caret scrolls it

Chromium scrolls overflow:hidden ancestors when a textarea caret moves out
of view (PageUp/PageDown in the prompt box, long prompts), shifting the whole
app up and hiding the title bar with no way to scroll back. Snap html/body/#root
back to zero on any root scroll event in the web, desktop, VS Code and mini-chat
apps.

Claude-Session: https://claude.ai/code/session_017TK5JAYDfT3Fotc23UEg98

packages/ui/src/App.tsx

@@ -20,6 +20,7 @@ import { useWebNotificationStream } from '@/hooks/useWebNotificationStream';
 import { useAgentMemorySync } from '@/hooks/useAgentMemorySync';
 import { usePwaInstallPrompt } from '@/hooks/usePwaInstallPrompt';
 import { useWindowTitle } from '@/hooks/useWindowTitle';
+import { useRootScrollLock } from '@/hooks/useRootScrollLock';
 import { useConfigStore } from '@/stores/useConfigStore';
 import { isDesktopLocalOriginActive, isDesktopShell, restartDesktopApp, invokeDesktop } from '@/lib/desktop';
 import {
@@ -717,6 +718,8 @@ function App({ apis }: AppProps) {
 
   useWindowTitle();
 
+  useRootScrollLock();
+
   useRouter();
 
   const handleToggleMemoryDebug = React.useCallback(() => {