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/transcription/transcriptionMarkers.ts

@@ -0,0 +1,25 @@
+export type TextSegment = { type: 'text' | 'marker'; text: string };
+
+const MARKER_PATTERN = /(\[unleserlich\]|\[\.{3}\])/g;
+
+export function splitByMarkers(input: string): TextSegment[] {
+	if (!input) return [];
+
+	const segments: TextSegment[] = [];
+	let lastIndex = 0;
+
+	for (const match of input.matchAll(MARKER_PATTERN)) {
+		const matchStart = match.index;
+		if (matchStart > lastIndex) {
+			segments.push({ type: 'text', text: input.slice(lastIndex, matchStart) });
+		}
+		segments.push({ type: 'marker', text: match[0] });
+		lastIndex = matchStart + match[0].length;
+	}
+
+	if (lastIndex < input.length) {
+		segments.push({ type: 'text', text: input.slice(lastIndex) });
+	}
+
+	return segments;
+}