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/apps/ElectronMiniChatApp.tsx

@@ -8,6 +8,7 @@ import { MiniChatLayout } from '@/components/mini-chat/MiniChatLayout';
 import { AppLinkConfirmDialog } from '@/components/chat/AppLinkConfirmDialog';
 import { usePushVisibilityBeacon } from '@/hooks/usePushVisibilityBeacon';
 import { useWindowTitle } from '@/hooks/useWindowTitle';
+import { useRootScrollLock } from '@/hooks/useRootScrollLock';
 import { opencodeClient } from '@/lib/opencode/client';
 import type { RuntimeAPIs } from '@/lib/api/types';
 import { useDirectoryStore } from '@/stores/useDirectoryStore';
@@ -318,6 +319,7 @@ export function ElectronMiniChatApp({ apis }: ElectronMiniChatAppProps) {
   useMiniChatKeyboardShortcuts();
   usePushVisibilityBeacon({ enabled: true });
   useWindowTitle();
+  useRootScrollLock();
 
   return (
     <ErrorBoundary>

packages/ui/src/apps/VSCodeApp.tsx

@@ -14,6 +14,7 @@ import { usePushVisibilityBeacon } from '@/hooks/usePushVisibilityBeacon';
 import { useGlobalSessionsPolling } from '@/hooks/useGlobalSessionsPolling';
 import { useRouter } from '@/hooks/useRouter';
 import { useWindowTitle } from '@/hooks/useWindowTitle';
+import { useRootScrollLock } from '@/hooks/useRootScrollLock';
 import { opencodeClient } from '@/lib/opencode/client';
 import type { RuntimeAPIs } from '@/lib/api/types';
 import { runtimeFetch } from '@/lib/runtime-fetch';
@@ -57,6 +58,7 @@ export function VSCodeApp({ apis }: VSCodeAppProps) {
   useAppFontEffects();
   usePushVisibilityBeacon({ enabled: true });
   useWindowTitle();
+  useRootScrollLock();
   useRouter();
   useGlobalSessionsPolling(panelType !== 'agentManager');