Support concurrent chats (#1478)

Fixes #212 


<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> Add concurrent chat support with per-chat state, chat activity UI, IPC
per-chat handling, and accompanying tests.
> 
> - **Frontend (Chat concurrency)**
> - Replace global chat atoms with per-chat maps:
`chatMessagesByIdAtom`, `isStreamingByIdAtom`, `chatErrorByIdAtom`,
`chatStreamCountByIdAtom`, `recentStreamChatIdsAtom`.
> - Update `ChatPanel`, `ChatInput`, `MessagesList`,
`DyadMarkdownParser`, and `useVersions` to read/write per-chat state.
> - Add `useSelectChat` to centralize selecting/navigating chats; wire
into `ChatList`.
> - **UI**
> - Add chat activity popover: `ChatActivityButton` and list; integrate
into `preview_panel/ActionHeader` (renamed from `PreviewHeader`) and
swap in `TitleBar`.
> - **IPC/Main**
> - Send error payloads with `chatId` on `chat:response:error`; update
`ipc_client` to route errors per chat.
> - Persist streaming partial assistant content periodically; improve
cancellation/end handling.
> - Make `FileUploadsState` per-chat (`addFileUpload({chatId,fileId},
...)`, `clear(chatId)`, `getFileUploadsForChat(chatId)`); update
handlers/processors accordingly.
> - **Testing**
> - Add e2e `concurrent_chat.spec.ts` and snapshots; extend helpers
(`snapshotMessages` timeout, chat activity helpers).
> - Fake LLM server: support `tc=` with options, optional sleep delay to
simulate concurrency.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
9035f30b73a1f2e5a366a0cac1c63411742b16f3. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

src/components/ChatList.tsx

@@ -28,11 +28,12 @@ import { RenameChatDialog } from "@/components/chat/RenameChatDialog";
 import { DeleteChatDialog } from "@/components/chat/DeleteChatDialog";
 
 import { ChatSearchDialog } from "./ChatSearchDialog";
+import { useSelectChat } from "@/hooks/useSelectChat";
 
 export function ChatList({ show }: { show?: boolean }) {
   const navigate = useNavigate();
   const [selectedChatId, setSelectedChatId] = useAtom(selectedChatIdAtom);
-  const [selectedAppId, setSelectedAppId] = useAtom(selectedAppIdAtom);
+  const [selectedAppId] = useAtom(selectedAppIdAtom);
   const [, setIsDropdownOpen] = useAtom(dropdownOpenAtom);
 
   const { chats, loading, refreshChats } = useChats(selectedAppId);
@@ -51,6 +52,7 @@ export function ChatList({ show }: { show?: boolean }) {
 
   // search dialog state
   const [isSearchDialogOpen, setIsSearchDialogOpen] = useState(false);
+  const { selectChat } = useSelectChat();
 
   // Update selectedChatId when route changes
   useEffect(() => {
@@ -74,13 +76,8 @@ export function ChatList({ show }: { show?: boolean }) {
     chatId: number;
     appId: number;
   }) => {
-    setSelectedChatId(chatId);
-    setSelectedAppId(appId);
+    selectChat({ chatId, appId });
     setIsSearchDialogOpen(false);
-    navigate({
-      to: "/chat",
-      search: { id: chatId },
-    });
   };
 
   const handleNewChat = async () => {