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 -->

testing/fake-llm-server/chatCompletionHandler.ts

@@ -6,7 +6,7 @@ import { CANNED_MESSAGE, createStreamChunk } from ".";
 let globalCounter = 0;
 
 export const createChatCompletionHandler =
-  (prefix: string) => (req: Request, res: Response) => {
+  (prefix: string) => async (req: Request, res: Response) => {
     const { stream = false, messages = [] } = req.body;
     console.log("* Received messages", messages);
 
@@ -42,6 +42,14 @@ DYAD_ATTACHMENT_0
       messageContent += "\n\n" + generateDump(req);
     }
 
+    if (
+      lastMessage &&
+      typeof lastMessage.content === "string" &&
+      lastMessage.content.includes("[sleep=medium]")
+    ) {
+      await new Promise((resolve) => setTimeout(resolve, 10_000));
+    }
+
     // TS auto-fix prefixes
     if (
       lastMessage &&
@@ -145,7 +153,8 @@ export default Index;
       typeof lastMessage.content === "string" &&
       lastMessage.content.startsWith("tc=")
     ) {
-      const testCaseName = lastMessage.content.slice(3); // Remove "tc=" prefix
+      const testCaseName = lastMessage.content.slice(3).split("[")[0].trim(); // Remove "tc=" prefix
+      console.error(`* Loading test case: ${testCaseName}`);
       const testFilePath = path.join(
         __dirname,
         "..",
@@ -162,7 +171,7 @@ export default Index;
           messageContent = fs.readFileSync(testFilePath, "utf-8");
           console.log(`* Loaded test case: ${testCaseName}`);
         } else {
-          console.log(`* Test case file not found: ${testFilePath}`);
+          console.error(`* Test case file not found: ${testFilePath}`);
           messageContent = `Error: Test case file not found: ${testCaseName}.md`;
         }
       } catch (error) {