fix(web): add dismiss button to thread error banner

The thread error banner had no way to be dismissed. Users hitting
unrecoverable errors (e.g. missing gh CLI during PR creation) were
stuck with a persistent error banner and no way to clear it.

Add a dismiss (X) button to ThreadErrorBanner using the existing
AlertAction component, wired to clear the thread error via
setThreadError(id, null).

Closes #496

Author: binbandit

apps/web/src/components/ChatView.tsx

@@ -123,7 +123,7 @@ import {
 } from "../keybindings";
 import ChatMarkdown from "./ChatMarkdown";
 import ThreadTerminalDrawer from "./ThreadTerminalDrawer";
-import { Alert, AlertDescription, AlertTitle } from "./ui/alert";
+import { Alert, AlertAction, AlertDescription, AlertTitle } from "./ui/alert";
 import {
   BotIcon,
   ChevronDownIcon,
@@ -3434,7 +3434,10 @@ export default function ChatView({ threadId }: ChatViewProps) {
 
       {/* Error banner */}
       <ProviderHealthBanner status={activeProviderStatus} />
-      <ThreadErrorBanner error={activeThread.error} />
+      <ThreadErrorBanner
+        error={activeThread.error}
+        onDismiss={() => setThreadError(activeThread.id, null)}
+      />
       <PlanModePanel activePlan={activePlan} />
 
       {/* Messages */}
@@ -4095,7 +4098,13 @@ const ChatHeader = memo(function ChatHeader({
   );
 });
 
-const ThreadErrorBanner = memo(function ThreadErrorBanner({ error }: { error: string | null }) {
+const ThreadErrorBanner = memo(function ThreadErrorBanner({
+  error,
+  onDismiss,
+}: {
+  error: string | null;
+  onDismiss?: () => void;
+}) {
   if (!error) return null;
   return (
     <div className="pt-3 mx-auto max-w-3xl">
@@ -4104,6 +4113,18 @@ const ThreadErrorBanner = memo(function ThreadErrorBanner({ error }: { error: st
         <AlertDescription className="line-clamp-3" title={error}>
           {error}
         </AlertDescription>
+        {onDismiss && (
+          <AlertAction>
+            <button
+              type="button"
+              aria-label="Dismiss error"
+              className="inline-flex size-6 items-center justify-center rounded-md text-destructive/60 transition-colors hover:text-destructive"
+              onClick={onDismiss}
+            >
+              <XIcon className="size-3.5" />
+            </button>
+          </AlertAction>
+        )}
       </Alert>
     </div>
   );