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/preview_panel/ActionHeader.tsx

@@ -12,6 +12,7 @@ import {
   Wrench,
   Globe,
 } from "lucide-react";
+import { ChatActivityButton } from "@/components/chat/ChatActivity";
 import { motion } from "framer-motion";
 import { useEffect, useRef, useState, useCallback } from "react";
 
@@ -44,7 +45,7 @@ const BUTTON_CLASS_NAME =
   "no-app-region-drag cursor-pointer relative flex items-center gap-1 px-2 py-1 rounded-md text-[13px] font-medium z-10 hover:bg-[var(--background)]";
 
 // Preview Header component with preview mode toggle
-export const PreviewHeader = () => {
+export const ActionHeader = () => {
   const [previewMode, setPreviewMode] = useAtom(previewModeAtom);
   const [isPreviewOpen, setIsPreviewOpen] = useAtom(isPreviewOpenAtom);
   const selectedAppId = useAtomValue(selectedAppIdAtom);
@@ -58,7 +59,7 @@ export const PreviewHeader = () => {
   const { problemReport } = useCheckProblems(selectedAppId);
   const { restartApp, refreshAppIframe } = useRunApp();
 
-  const isCompact = windowWidth < 860;
+  const isCompact = windowWidth < 888;
 
   // Track window width
   useEffect(() => {
@@ -257,7 +258,9 @@ export const PreviewHeader = () => {
             "publish-mode-button",
           )}
         </div>
-        <div className="flex items-center">
+        {/* Chat activity bell */}
+        <div className="flex items-center gap-1">
+          <ChatActivityButton />
           <DropdownMenu>
             <DropdownMenuTrigger asChild>
               <button