refactor: move document transcription, annotation, viewer sub-packages

- transcription/: TranscriptionBlock, Column, EditView, PanelHeader, ReadView,
  Section + transcriptionMarkers, blockConflictMerge, saveBlockWithConflictRetry
  + useBlockAutoSave, useBlockDragDrop hooks
- annotation/: AnnotationLayer, AnnotationShape, AnnotationEditOverlay
- viewer/: PdfViewer, PdfControls + useFileLoader, usePdfRenderer hooks

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

frontend/src/lib/document/DocumentViewer.svelte

@@ -1,6 +1,6 @@
 <script lang="ts">
 import { m } from '$lib/paraglide/messages.js';
-import PdfViewer from '$lib/components/PdfViewer.svelte';
+import PdfViewer from '$lib/document/viewer/PdfViewer.svelte';
 
 type Doc = {
 	id: string;