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/app/TitleBar.tsx

@@ -20,7 +20,7 @@ import {
   TooltipContent,
   TooltipTrigger,
 } from "@/components/ui/tooltip";
-import { PreviewHeader } from "@/components/preview_panel/PreviewHeader";
+import { ActionHeader } from "@/components/preview_panel/ActionHeader";
 
 export const TitleBar = () => {
   const [selectedAppId] = useAtom(selectedAppIdAtom);
@@ -97,7 +97,7 @@ export const TitleBar = () => {
         {/* Preview Header */}
         {location.pathname === "/chat" && (
           <div className="flex-1 flex justify-end">
-            <PreviewHeader />
+            <ActionHeader />
           </div>
         )}