fix(chat-switch): remount ConversationPage per conversation id

apps/desktop/src/App.tsx

@@ -1,5 +1,5 @@
 import { lazy, Suspense } from 'react';
-import { HashRouter, Navigate, Outlet, Route, Routes } from 'react-router-dom';
+import { HashRouter, Navigate, Outlet, Route, Routes, useParams } from 'react-router-dom';
 
 import { AppShell } from './components/AppShell';
 import { CrashToast } from './components/CrashToast';
@@ -60,6 +60,18 @@ function RouteBoundary({ scope }: { scope: string }) {
   );
 }
 
+// Forces a fresh `ConversationPage` instance per `:id` so React unmounts
+// the previous conversation entirely on switch. Without this, the same
+// component instance handles every conversation, which leaks state
+// between chats (messages, scroll position, composer drafts) for one
+// render frame and gives the "flicker" we're trying to remove.
+// `useConversationMessages` re-hydrates from `messageMemoryCache` on the
+// fresh mount so previously-visited chats still render instantly.
+function ConversationRoute() {
+  const { id } = useParams<{ id: string }>();
+  return <ConversationPage key={id ?? '__no_id__'} />;
+}
+
 export function App() {
   return (
     <ErrorBoundary scope="root">
@@ -114,7 +126,7 @@ export function App() {
                                 path=":id"
                                 element={
                                   <ErrorBoundary scope="conversation">
-                                    <ConversationPage />
+                                    <ConversationRoute />
                                   </ErrorBoundary>
                                 }
                               />