feat(web): add ApiErrorBanner retry-capable error card

web/src/lib/components/ApiErrorBanner.svelte

@@ -0,0 +1,22 @@
+<script lang="ts">
+  import type { ApiError } from '$lib/api/client';
+
+  let {
+    error,
+    onRetry
+  }: { error: unknown; onRetry: () => void } = $props();
+
+  const apiErr = $derived(error as ApiError | undefined);
+  const message = $derived(apiErr?.message ?? 'Something went wrong.');
+</script>
+
+<div role="alert" class="rounded border border-danger/40 bg-surface p-4">
+  <p class="text-text-primary">{message}</p>
+  <button
+    type="button"
+    class="mt-2 rounded bg-accent px-3 py-1 text-sm text-background"
+    onclick={onRetry}
+  >
+    Try again
+  </button>
+</div>

web/src/lib/components/ApiErrorBanner.test.ts

@@ -0,0 +1,32 @@
+import { describe, expect, test, vi } from 'vitest';
+import { render, screen, fireEvent } from '@testing-library/svelte';
+import ApiErrorBanner from './ApiErrorBanner.svelte';
+
+describe('ApiErrorBanner', () => {
+  test('renders error.message when present', () => {
+    render(ApiErrorBanner, {
+      props: {
+        error: { code: 'server_error', message: 'database down', status: 500 },
+        onRetry: () => {}
+      }
+    });
+    expect(screen.getByRole('alert')).toHaveTextContent('database down');
+  });
+
+  test('falls back to generic text when error lacks message', () => {
+    render(ApiErrorBanner, { props: { error: undefined, onRetry: () => {} } });
+    expect(screen.getByRole('alert')).toHaveTextContent(/something went wrong/i);
+  });
+
+  test('clicking the button calls onRetry', async () => {
+    const onRetry = vi.fn();
+    render(ApiErrorBanner, {
+      props: {
+        error: { code: 'server_error', message: 'x', status: 500 },
+        onRetry
+      }
+    });
+    await fireEvent.click(screen.getByRole('button', { name: /try again/i }));
+    expect(onRetry).toHaveBeenCalledTimes(1);
+  });
+});