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/ipc/ipc_client.ts

@@ -189,15 +189,27 @@ export class IpcClient {
       }
     });
 
-    this.ipcRenderer.on("chat:response:error", (error) => {
+    this.ipcRenderer.on("chat:response:error", (payload) => {
       console.debug("chat:response:error");
-      if (typeof error === "string") {
-        for (const [chatId, callbacks] of this.chatStreams.entries()) {
+      if (
+        payload &&
+        typeof payload === "object" &&
+        "chatId" in payload &&
+        "error" in payload
+      ) {
+        const { chatId, error } = payload as { chatId: number; error: string };
+        const callbacks = this.chatStreams.get(chatId);
+        if (callbacks) {
           callbacks.onError(error);
           this.chatStreams.delete(chatId);
+        } else {
+          console.warn(
+            `[IPC] No callbacks found for chat ${chatId} on error`,
+            this.chatStreams,
+          );
         }
       } else {
-        console.error("[IPC] Invalid error data received:", error);
+        console.error("[IPC] Invalid error data received:", payload);
       }
     });