perf: instant proxy mode switching via optimistic UI update

Replace slow accounts.refresh() (fetches quota from chatgpt.com per account)
with local state patch for immediate UI response.

shared/hooks/use-accounts.ts

@@ -128,6 +128,10 @@ export function useAccounts() {
     [loadAccounts]
   );
 
+  const patchLocal = useCallback((accountId: string, patch: Partial<Account>) => {
+    setList((prev) => prev.map((a) => a.id === accountId ? { ...a, ...patch } : a));
+  }, []);
+
   return {
     list,
     loading,
@@ -137,6 +141,7 @@ export function useAccounts() {
     addInfo,
     addError,
     refresh: loadAccounts,
+    patchLocal,
     startAdd,
     submitRelay,
     deleteAccount,

web/src/App.tsx

@@ -62,8 +62,8 @@ function Dashboard() {
   const update = useUpdateMessage();
 
   const handleProxyChange = async (accountId: string, proxyId: string) => {
+    accounts.patchLocal(accountId, { proxyId });
     await proxies.assignProxy(accountId, proxyId);
-    accounts.refresh();
   };
 
   return (