Disable send button while approval is pending (#1368)

Fixes #912 

This PR implements disabling send button while approval is pending and
addresses issue #912
    
<!-- This is an auto-generated description by cubic. -->
---

## Summary by cubic
Disable the chat send button while a proposal is awaiting approval, and
re-enable it after approve or reject. Prevents accidental messages
during pending changes. Addresses issue #912.

- **New Features**
- Track pending changes with isChangesPending based on the last
assistant message’s approvalState.
- Disable the send button when a proposal is pending (in addition to the
existing empty input check).
- Re-enable after approve/reject by refreshing the proposal and
messages.
  - Added Playwright e2e tests for both approve and reject flows.

<!-- End of auto-generated description by cubic. -->

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> Disable the chat send button when a code proposal is pending approval
and re-enable after approve/reject; add e2e coverage and update MCP
flow.
> 
> - **Frontend**
> - `ChatInput.tsx`: Read `messages` from `chatMessagesAtom` and derive
`disableSendButton` when the last assistant message (matching
`proposal.messageId`) has no `approvalState` and `proposal.type ===
"code-proposal"`.
> - Apply `disableSendButton` to the send button’s `disabled` condition
(in addition to empty input/attachments).
>   - Ensure proposal/messages refresh after approve/reject.
> - **Tests**
> - Add Playwright tests `e2e-tests/chat_input.spec.ts` to verify send
button disabled during pending proposal and re-enabled after approve or
reject.
> - Update `e2e-tests/mcp.spec.ts` to click "Approve" after granting
consent.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
b9b47bd6f547449cc5cf1d39a00e4e7fb5de1bcd. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

---------

Co-authored-by: Will Chen <willchen90@gmail.com>

src/components/chat/ChatInput.tsx

@@ -79,7 +79,7 @@ export function ChatInput({ chatId }: { chatId?: number }) {
   const [showError, setShowError] = useState(true);
   const [isApproving, setIsApproving] = useState(false); // State for approving
   const [isRejecting, setIsRejecting] = useState(false); // State for rejecting
-  const [, setMessages] = useAtom<Message[]>(chatMessagesAtom);
+  const [messages, setMessages] = useAtom<Message[]>(chatMessagesAtom);
   const setIsPreviewOpen = useSetAtom(isPreviewOpenAtom);
   const [showTokenBar, setShowTokenBar] = useAtom(showTokenBarAtom);
   const [selectedComponent, setSelectedComponent] = useAtom(
@@ -108,6 +108,14 @@ export function ChatInput({ chatId }: { chatId?: number }) {
   } = useProposal(chatId);
   const { proposal, messageId } = proposalResult ?? {};
 
+  const lastMessage = messages.at(-1);
+  const disableSendButton =
+    lastMessage?.role === "assistant" &&
+    !lastMessage.approvalState &&
+    !!proposal &&
+    proposal.type === "code-proposal" &&
+    messageId === lastMessage.id;
+
   useEffect(() => {
     if (error) {
       setShowError(true);
@@ -214,7 +222,6 @@ export function ChatInput({ chatId }: { chatId?: number }) {
       setError((err as Error)?.message || "An error occurred while rejecting");
     } finally {
       setIsRejecting(false);
-
       // Keep same as handleApprove
       refreshProposal();
       fetchChatMessages();
@@ -307,7 +314,10 @@ export function ChatInput({ chatId }: { chatId?: number }) {
             ) : (
               <button
                 onClick={handleSubmit}
-                disabled={!inputValue.trim() && attachments.length === 0}
+                disabled={
+                  (!inputValue.trim() && attachments.length === 0) ||
+                  disableSendButton
+                }
                 className="px-2 py-2 mt-1 mr-1 hover:bg-(--background-darkest) text-(--sidebar-accent-fg) rounded-lg disabled:opacity-50"
                 title="Send message"
               >