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/__mocks__/navigatingStore.ts

@@ -0,0 +1,3 @@
+import { writable } from 'svelte/store';
+
+export const navigatingStore = writable<unknown | null>(null);