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.ts

@@ -0,0 +1,10 @@
+export const ALLOWED_TYPES = new Set(['application/pdf', 'image/jpeg', 'image/png', 'image/tiff']);
+export const MAX_SIZE_BYTES = 50 * 1024 * 1024;
+
+export type FileValidationError = 'type' | 'size';
+
+export function validateFile(file: File): FileValidationError | null {
+	if (!ALLOWED_TYPES.has(file.type)) return 'type';
+	if (file.size > MAX_SIZE_BYTES) return 'size';
+	return null;
+}