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/validateFile.spec.ts

@@ -0,0 +1,36 @@
+import { describe, it, expect } from 'vitest';
+import { validateFile, MAX_SIZE_BYTES } from './validateFile';
+
+function makeFile(type: string, size: number): File {
+	return new File(['x'.repeat(Math.min(size, 100))], 'test.file', { type });
+}
+
+describe('validateFile', () => {
+	it('returns null for a valid PDF under 50 MB', () => {
+		const file = makeFile('application/pdf', 1024);
+		expect(validateFile(file)).toBeNull();
+	});
+
+	it('returns null for a valid JPEG', () => {
+		expect(validateFile(makeFile('image/jpeg', 1024))).toBeNull();
+	});
+
+	it('returns null for a valid PNG', () => {
+		expect(validateFile(makeFile('image/png', 1024))).toBeNull();
+	});
+
+	it('returns null for a valid TIFF', () => {
+		expect(validateFile(makeFile('image/tiff', 1024))).toBeNull();
+	});
+
+	it('returns "type" for an unsupported MIME type', () => {
+		const file = makeFile('text/plain', 100);
+		expect(validateFile(file)).toBe('type');
+	});
+
+	it('returns "size" for a file exceeding 50 MB', () => {
+		const oversized = new File(['x'], 'big.pdf', { type: 'application/pdf' });
+		Object.defineProperty(oversized, 'size', { value: MAX_SIZE_BYTES + 1 });
+		expect(validateFile(oversized)).toBe('size');
+	});
+});