feat(bulk-upload): add BulkDropZone component

Full-panel drop target that supports multi-file selection via drag-and-drop
or file picker. Fires onFilesAdded callback with the full File array.

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

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

@@ -0,0 +1,39 @@
+import { describe, it, expect, vi, afterEach } from 'vitest';
+import { cleanup, render } from 'vitest-browser-svelte';
+import { page, userEvent } from 'vitest/browser';
+import BulkDropZone from './BulkDropZone.svelte';
+
+afterEach(cleanup);
+
+describe('BulkDropZone', () => {
+	it('file input has multiple attribute', async () => {
+		const { container } = render(BulkDropZone, { onFilesAdded: vi.fn() });
+		const input = container.querySelector('input[type="file"]');
+		expect(input).not.toBeNull();
+		expect(input?.hasAttribute('multiple')).toBe(true);
+	});
+
+	it('fires onFilesAdded with selected files when 3 files are picked via input', async () => {
+		const onFilesAdded = vi.fn();
+		render(BulkDropZone, { onFilesAdded });
+
+		const files = [
+			new File(['a'], 'a.pdf', { type: 'application/pdf' }),
+			new File(['b'], 'b.pdf', { type: 'application/pdf' }),
+			new File(['c'], 'c.pdf', { type: 'application/pdf' })
+		];
+
+		const input = page.getByRole('button', { name: /Dateien auswählen/i });
+		await userEvent.upload(input, files);
+
+		expect(onFilesAdded).toHaveBeenCalledOnce();
+		const received: File[] = onFilesAdded.mock.calls[0][0];
+		expect(received).toHaveLength(3);
+		expect(received.map((f) => f.name)).toEqual(['a.pdf', 'b.pdf', 'c.pdf']);
+	});
+
+	it('shows drop hint text', async () => {
+		render(BulkDropZone, { onFilesAdded: vi.fn() });
+		await expect.element(page.getByText(/hier ablegen/i)).toBeInTheDocument();
+	});
+});