feat(bulk-upload): add FileSwitcherStrip component

Horizontal chip strip for switching between files in a bulk upload session.
Supports keyboard navigation (arrow keys cycle within the strip), error state
chips, and onSelect/onRemove callbacks.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/components/document/FileSwitcherStrip.svelte.spec.ts

@@ -0,0 +1,97 @@
+import { describe, it, expect, vi, afterEach } from 'vitest';
+import { cleanup, render } from 'vitest-browser-svelte';
+import { page, userEvent } from 'vitest/browser';
+import FileSwitcherStrip from './FileSwitcherStrip.svelte';
+
+afterEach(cleanup);
+
+export interface FileEntry {
+	id: string;
+	file: File;
+	title: string;
+	status: 'idle' | 'error';
+}
+
+function makeFiles(n: number): FileEntry[] {
+	return Array.from({ length: n }, (_, i) => ({
+		id: `id-${i}`,
+		file: new File([''], `file${i}.pdf`),
+		title: `File ${i}`,
+		status: 'idle' as const
+	}));
+}
+
+describe('FileSwitcherStrip', () => {
+	it('renders N chips for N files', async () => {
+		const files = makeFiles(4);
+		render(FileSwitcherStrip, {
+			files,
+			activeId: files[0].id,
+			onSelect: vi.fn(),
+			onRemove: vi.fn()
+		});
+		const chips = page.getByRole('listitem');
+		await expect.element(chips.nth(0)).toBeInTheDocument();
+		await expect.element(chips.nth(3)).toBeInTheDocument();
+	});
+
+	it('active chip has aria-current="true"', async () => {
+		const files = makeFiles(3);
+		const { container } = render(FileSwitcherStrip, {
+			files,
+			activeId: files[1].id,
+			onSelect: vi.fn(),
+			onRemove: vi.fn()
+		});
+		const activeBtn = container.querySelector('[aria-current="true"]');
+		expect(activeBtn).not.toBeNull();
+		expect(activeBtn?.textContent).toContain('File 1');
+	});
+
+	it('clicking a chip fires onSelect with its id', async () => {
+		const files = makeFiles(3);
+		const onSelect = vi.fn();
+		const { container } = render(FileSwitcherStrip, {
+			files,
+			activeId: files[0].id,
+			onSelect,
+			onRemove: vi.fn()
+		});
+		const chip = container.querySelector('[data-chip-id="id-2"]') as HTMLElement;
+		expect(chip).not.toBeNull();
+		chip.click();
+		expect(onSelect).toHaveBeenCalledWith('id-2');
+	});
+
+	it('error chip has aria-label containing warning indicator', async () => {
+		const files: FileEntry[] = [
+			{ id: 'e1', file: new File([''], 'bad.pdf'), title: 'Bad file', status: 'error' }
+		];
+		const { container } = render(FileSwitcherStrip, {
+			files,
+			activeId: 'e1',
+			onSelect: vi.fn(),
+			onRemove: vi.fn()
+		});
+		const errBtn = container.querySelector('[data-status="error"]');
+		expect(errBtn).not.toBeNull();
+	});
+
+	it('ArrowRight moves focus to next chip without leaving strip', async () => {
+		const files = makeFiles(3);
+		const { container } = render(FileSwitcherStrip, {
+			files,
+			activeId: files[0].id,
+			onSelect: vi.fn(),
+			onRemove: vi.fn()
+		});
+		const firstBtn = container.querySelectorAll('[role="button"]')[0] as HTMLElement;
+		firstBtn.focus();
+		await userEvent.keyboard('{ArrowRight}');
+		const focused = document.activeElement;
+		expect(focused).not.toBe(firstBtn);
+		// The new focused element should still be inside the strip
+		const strip = container.querySelector('[data-testid="file-switcher-strip"]');
+		expect(strip?.contains(focused)).toBe(true);
+	});
+});