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 -->

e2e-tests/concurrent_chat.spec.ts

@@ -0,0 +1,25 @@
+import { test } from "./helpers/test_helper";
+import { expect } from "@playwright/test";
+
+test("concurrent chat", async ({ po }) => {
+  await po.setUp();
+  await po.sendPrompt("tc=chat1 [sleep=medium]", {
+    skipWaitForCompletion: true,
+  });
+  // Need a short wait otherwise the click on Apps tab is ignored.
+  await po.sleep(2_000);
+
+  await po.goToAppsTab();
+  await po.sendPrompt("tc=chat2");
+  await po.snapshotMessages();
+  await po.clickChatActivityButton();
+
+  // Chat #1 will be the last in the list
+  expect(
+    await po.page.getByTestId(`chat-activity-list-item-1`).textContent(),
+  ).toContain("Chat #1");
+  await po.page.getByTestId(`chat-activity-list-item-1`).click();
+  await po.snapshotMessages({ timeout: 12_000 });
+
+  //
+});