fix: eliminate layout shift when switching between Claude and Codex

Fixed dual-source jitter issue:
1. Horizontal shift: Use overflow-y-scroll to force scrollbar gutter
2. Vertical jump: Use keepPreviousData to maintain content during app switch

src/App.tsx

@@ -145,7 +145,7 @@ function App() {
         </div>
       </header>
 
-      <main className="flex-1 overflow-y-auto">
+      <main className="flex-1 overflow-y-scroll">
         <div className="mx-auto max-w-4xl px-6 py-6">
           <ProviderList
             providers={providers}

src/lib/query/queries.ts

@@ -1,4 +1,4 @@
-import { useQuery, type UseQueryResult } from "@tanstack/react-query";
+import { useQuery, type UseQueryResult, keepPreviousData } from "@tanstack/react-query";
 import { providersApi, settingsApi, usageApi, type AppType } from "@/lib/api";
 import type { Provider, Settings, UsageResult } from "@/types";
 
@@ -35,6 +35,7 @@ export const useProvidersQuery = (
 ): UseQueryResult<ProvidersQueryData> => {
   return useQuery({
     queryKey: ["providers", appType],
+    placeholderData: keepPreviousData,
     queryFn: async () => {
       let providers: Record<string, Provider> = {};
       let currentProviderId = "";