fix(ai-chat): improve error banner layout

The AI chat error Alert stranded the warning icon in the top-left while
the detail text hung indented under the heading, wrapping to 3 narrow
lines with empty space below. Switch to a "full-width detail" layout
(icon + bold heading on the first row, detail spanning full width below)
and extract the markup, previously duplicated in ChatThread and
MessageItem, into a single shared ChatErrorAlert component.

- add apps/client/src/features/ai-chat/components/chat-error-alert.tsx
- use it for the live stream error in chat-thread.tsx (mb="xs")
- use it for the persisted history error in message-item.tsx (mt={4})
- heading/icon use the adaptive --mantine-color-red-light-color so the
  banner stays correct in dark mode

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

apps/client/src/features/ai-chat/components/chat-error-alert.tsx

@@ -0,0 +1,38 @@
+import { Alert, Group, Text, type AlertProps } from "@mantine/core";
+import { IconAlertTriangle } from "@tabler/icons-react";
+
+/**
+ * A classified AI chat error banner: a warning icon + bold heading on the first
+ * row, with the detail text spanning the full width below. Rendered for BOTH the
+ * live stream error (ChatThread) and a persisted assistant error (MessageItem),
+ * so this markup lives in one place. The detail is full-width (no hanging indent
+ * under the heading) so it wraps less and leaves no stranded icon / empty gap.
+ * The heading reuses Mantine's adaptive red "light" colour so it stays correct
+ * in dark mode. Layout-only props (mb/mt/...) are forwarded to the Alert root.
+ */
+interface ChatErrorAlertProps extends Omit<AlertProps, "title" | "children"> {
+  title: string;
+  detail: string;
+}
+
+export default function ChatErrorAlert({
+  title,
+  detail,
+  ...alertProps
+}: ChatErrorAlertProps) {
+  // Mantine's own "light" alert colour, adaptive across light/dark schemes.
+  const accent = "var(--mantine-color-red-light-color)";
+  return (
+    <Alert {...alertProps} variant="light" color="red" p="xs">
+      <Group gap={8} wrap="nowrap" align="center" mb={4}>
+        <IconAlertTriangle size={18} style={{ flex: "none", color: accent }} />
+        <Text fw={700} size="sm" lh={1.2} style={{ color: accent }}>
+          {title}
+        </Text>
+      </Group>
+      <Text size="sm" lh={1.4}>
+        {detail}
+      </Text>
+    </Alert>
+  );
+}

apps/client/src/features/ai-chat/components/chat-thread.tsx

@@ -1,13 +1,14 @@
 import { useCallback, useMemo, useRef, useState } from "react";
 import { generateId } from "ai";
-import { ActionIcon, Alert, Box, Group, Stack, Text } from "@mantine/core";
-import { IconAlertTriangle, IconClockHour4, IconX } from "@tabler/icons-react";
+import { ActionIcon, Box, Group, Stack, Text } from "@mantine/core";
+import { IconClockHour4, IconX } from "@tabler/icons-react";
 import { useTranslation } from "react-i18next";
 import { useChat, type UIMessage } from "@ai-sdk/react";
 import { DefaultChatTransport } from "ai";
 import MessageList from "@/features/ai-chat/components/message-list.tsx";
 import ChatInput from "@/features/ai-chat/components/chat-input.tsx";
 import RoleCards from "@/features/ai-chat/components/role-cards.tsx";
+import ChatErrorAlert from "@/features/ai-chat/components/chat-error-alert.tsx";
 import {
   IAiChatMessageRow,
   IAiRole,
@@ -277,15 +278,11 @@ export default function ChatThread({
       />
 
       {errorView && (
-        <Alert
-          variant="light"
-          color="red"
-          icon={<IconAlertTriangle size={16} />}
-          mb="xs"
+        <ChatErrorAlert
           title={errorView.title}
-        >
-          {errorView.detail}
-        </Alert>
+          detail={errorView.detail}
+          mb="xs"
+        />
       )}
 
       <Stack gap={0} className={classes.inputWrapper}>

apps/client/src/features/ai-chat/components/message-item.tsx

@@ -1,8 +1,8 @@
-import { Alert, Box, Text } from "@mantine/core";
-import { IconAlertTriangle } from "@tabler/icons-react";
+import { Box, Text } from "@mantine/core";
 import { useTranslation } from "react-i18next";
 import type { UIMessage } from "@ai-sdk/react";
 import ToolCallCard from "@/features/ai-chat/components/tool-call-card.tsx";
+import ChatErrorAlert from "@/features/ai-chat/components/chat-error-alert.tsx";
 import { ToolUiPart, isToolPart } from "@/features/ai-chat/utils/tool-parts.tsx";
 import { renderChatMarkdown } from "@/features/ai-chat/utils/markdown.ts";
 import { resolveAssistantName } from "@/features/ai-chat/utils/assistant-name.ts";
@@ -118,15 +118,11 @@ export default function MessageItem({
         // cause plus a one-line detail.
         const errorView = describeChatError(errorText, t);
         return (
-          <Alert
-            variant="light"
-            color="red"
-            icon={<IconAlertTriangle size={16} />}
-            mt={4}
+          <ChatErrorAlert
             title={errorView.title}
-          >
-            {errorView.detail}
-          </Alert>
+            detail={errorView.detail}
+            mt={4}
+          />
         );
       })()}
     </Box>