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/handlers/chat_stream_handlers.ts

@@ -206,7 +206,6 @@ export function registerChatStreamHandlers() {
   ipcMain.handle("chat:stream", async (event, req: ChatStreamParams) => {
     try {
       const fileUploadsState = FileUploadsState.getInstance();
-      fileUploadsState.initialize({ chatId: req.chatId });
 
       // Create an AbortController for this stream
       const abortController = new AbortController();
@@ -288,10 +287,13 @@ export function registerChatStreamHandlers() {
             // For upload-to-codebase, create a unique file ID and store the mapping
             const fileId = `DYAD_ATTACHMENT_${index}`;
 
-            fileUploadsState.addFileUpload(fileId, {
-              filePath,
-              originalName: attachment.name,
-            });
+            fileUploadsState.addFileUpload(
+              { chatId: req.chatId, fileId },
+              {
+                filePath,
+                originalName: attachment.name,
+              },
+            );
 
             // Add instruction for AI to use dyad-write tag
             attachmentInfo += `\n\nFile to upload to codebase: ${attachment.name} (file id: ${fileId})\n`;
@@ -793,10 +795,10 @@ This conversation includes one or more image attachments. When the user uploads
               const requestIdPrefix = isEngineEnabled
                 ? `[Request ID: ${dyadRequestId}] `
                 : "";
-              event.sender.send(
-                "chat:response:error",
-                `Sorry, there was an error from the AI: ${requestIdPrefix}${message}`,
-              );
+              event.sender.send("chat:response:error", {
+                chatId: req.chatId,
+                error: `Sorry, there was an error from the AI: ${requestIdPrefix}${message}`,
+              });
               // Clean up the abort controller
               activeStreams.delete(req.chatId);
             },
@@ -804,6 +806,8 @@ This conversation includes one or more image attachments. When the user uploads
           });
         };
 
+        let lastDbSaveAt = 0;
+
         const processResponseChunkUpdate = async ({
           fullResponse,
         }: {
@@ -823,6 +827,16 @@ This conversation includes one or more image attachments. When the user uploads
           }
           // Store the current partial response
           partialResponses.set(req.chatId, fullResponse);
+          // Save to DB (in case user is switching chats during the stream)
+          const now = Date.now();
+          if (now - lastDbSaveAt >= 150) {
+            await db
+              .update(messages)
+              .set({ content: fullResponse })
+              .where(eq(messages.id, placeholderAssistantMessage.id));
+
+            lastDbSaveAt = now;
+          }
 
           // Update the placeholder assistant message content in the messages array
           const currentMessages = [...updatedChat.messages];
@@ -1143,11 +1157,10 @@ ${problemReport.problems
           });
 
           if (status.error) {
-            safeSend(
-              event.sender,
-              "chat:response:error",
-              `Sorry, there was an error applying the AI's changes: ${status.error}`,
-            );
+            safeSend(event.sender, "chat:response:error", {
+              chatId: req.chatId,
+              error: `Sorry, there was an error applying the AI's changes: ${status.error}`,
+            });
           }
 
           // Signal that the stream has completed
@@ -1190,15 +1203,14 @@ ${problemReport.problems
       return req.chatId;
     } catch (error) {
       logger.error("Error calling LLM:", error);
-      safeSend(
-        event.sender,
-        "chat:response:error",
-        `Sorry, there was an error processing your request: ${error}`,
-      );
+      safeSend(event.sender, "chat:response:error", {
+        chatId: req.chatId,
+        error: `Sorry, there was an error processing your request: ${error}`,
+      });
       // Clean up the abort controller
       activeStreams.delete(req.chatId);
       // Clean up file uploads state on error
-      FileUploadsState.getInstance().clear();
+      FileUploadsState.getInstance().clear(req.chatId);
       return "error";
     }
   });
@@ -1222,6 +1234,11 @@ ${problemReport.problems
       updatedFiles: false,
     } satisfies ChatResponseEnd);
 
+    // Clean up uploads state for this chat
+    try {
+      FileUploadsState.getInstance().clear(chatId);
+    } catch {}
+
     return true;
   });
 }