refactor: move document domain core to lib/document/

Moves ~25 components, utils (search, filename, groupDocuments,
documentStatusLabel, validateFile), bulkSelection store, and
TranscriptionSection sub-component. Fixes broken relative imports.

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

frontend/src/lib/document/UploadZone.svelte.spec.ts

@@ -0,0 +1,25 @@
+import { describe, it, expect, afterEach } from 'vitest';
+import { cleanup, render } from 'vitest-browser-svelte';
+import { page } from 'vitest/browser';
+import UploadZone from './UploadZone.svelte';
+
+afterEach(cleanup);
+
+describe('UploadZone', () => {
+	it('renders an SVG arrow icon (not a Unicode character) in idle state', async () => {
+		const { container } = render(UploadZone, {
+			filename: 'test.pdf',
+			isUploading: false,
+			error: null
+		});
+		// The icon must be an SVG element, not the raw "↑" text
+		const svg = container.querySelector('.upload-zone-icon svg');
+		expect(svg).not.toBeNull();
+		expect(container.textContent).not.toContain('↑');
+	});
+
+	it('shows the filename in idle state', async () => {
+		render(UploadZone, { filename: 'my-document.pdf', isUploading: false, error: null });
+		await expect.element(page.getByText('my-document.pdf')).toBeInTheDocument();
+	});
+});