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/blockConflictMerge.spec.ts

@@ -0,0 +1,128 @@
+import { describe, it, expect } from 'vitest';
+import { BlockConflictResolvedError, mergeBlockOnConflict } from './blockConflictMerge';
+import type { PersonMention, TranscriptionBlockData } from '$lib/types';
+
+const baseBlock: TranscriptionBlockData = {
+	id: 'b1',
+	annotationId: 'a1',
+	documentId: 'd1',
+	text: 'old text from server',
+	label: null,
+	sortOrder: 0,
+	version: 7,
+	source: 'MANUAL',
+	reviewed: false,
+	mentionedPersons: []
+};
+
+describe('mergeBlockOnConflict', () => {
+	it('keeps the local unsaved text — never overwritten by server text (B12b)', () => {
+		const merged = mergeBlockOnConflict({
+			serverBlock: { ...baseBlock, text: 'server-side text' },
+			localText: 'transcriber unsaved input',
+			localMentions: []
+		});
+		expect(merged.text).toBe('transcriber unsaved input');
+	});
+
+	it('takes server-side displayName for personIds present on both sides (rename win)', () => {
+		const localMentions: PersonMention[] = [
+			{ personId: 'p-aug', displayName: 'Auguste Raddatz' } // stale: server renamed her
+		];
+		const serverMentions: PersonMention[] = [
+			{ personId: 'p-aug', displayName: 'Augusta Raddatz' } // post-rename
+		];
+		const merged = mergeBlockOnConflict({
+			serverBlock: { ...baseBlock, mentionedPersons: serverMentions },
+			localText: '@Augusta Raddatz',
+			localMentions
+		});
+		expect(merged.mentionedPersons).toEqual([
+			{ personId: 'p-aug', displayName: 'Augusta Raddatz' }
+		]);
+	});
+
+	it('keeps local-only mentions added since last save', () => {
+		const localMentions: PersonMention[] = [
+			{ personId: 'p-anna', displayName: 'Anna Schmidt' } // typed since last save
+		];
+		const merged = mergeBlockOnConflict({
+			serverBlock: { ...baseBlock, mentionedPersons: [] },
+			localText: '@Anna Schmidt',
+			localMentions
+		});
+		expect(merged.mentionedPersons).toContainEqual({
+			personId: 'p-anna',
+			displayName: 'Anna Schmidt'
+		});
+	});
+
+	it('returns a union of personIds when local and server diverge', () => {
+		const localMentions: PersonMention[] = [{ personId: 'p-anna', displayName: 'Anna Schmidt' }];
+		const serverMentions: PersonMention[] = [{ personId: 'p-aug', displayName: 'Augusta Raddatz' }];
+		const merged = mergeBlockOnConflict({
+			serverBlock: { ...baseBlock, mentionedPersons: serverMentions },
+			localText: '@Augusta Raddatz und @Anna Schmidt',
+			localMentions
+		});
+		expect(merged.mentionedPersons).toHaveLength(2);
+		expect(merged.mentionedPersons).toContainEqual({
+			personId: 'p-aug',
+			displayName: 'Augusta Raddatz'
+		});
+		expect(merged.mentionedPersons).toContainEqual({
+			personId: 'p-anna',
+			displayName: 'Anna Schmidt'
+		});
+	});
+
+	it('carries server version forward so the next save sends the latest revision', () => {
+		const merged = mergeBlockOnConflict({
+			serverBlock: { ...baseBlock, version: 42 },
+			localText: 'x',
+			localMentions: []
+		});
+		expect(merged.version).toBe(42);
+	});
+
+	it('carries server-only mention array through when local has none', () => {
+		const merged = mergeBlockOnConflict({
+			serverBlock: {
+				...baseBlock,
+				mentionedPersons: [
+					{ personId: 'p-aug', displayName: 'Augusta Raddatz' },
+					{ personId: 'p-anna', displayName: 'Anna Schmidt' }
+				]
+			},
+			localText: 'x',
+			localMentions: []
+		});
+		expect(merged.mentionedPersons).toHaveLength(2);
+	});
+
+	it('carries other server fields (sortOrder, reviewed, updatedAt) forward', () => {
+		const merged = mergeBlockOnConflict({
+			serverBlock: {
+				...baseBlock,
+				sortOrder: 9,
+				reviewed: true,
+				updatedAt: '2026-04-29T10:00:00Z'
+			},
+			localText: 'x',
+			localMentions: []
+		});
+		expect(merged.sortOrder).toBe(9);
+		expect(merged.reviewed).toBe(true);
+		expect(merged.updatedAt).toBe('2026-04-29T10:00:00Z');
+	});
+});
+
+describe('BlockConflictResolvedError', () => {
+	it('is an Error with code = CONFLICT_RESOLVED', () => {
+		const err = new BlockConflictResolvedError('block-1');
+		expect(err).toBeInstanceOf(Error);
+		expect(err.code).toBe('CONFLICT_RESOLVED');
+		expect(err.name).toBe('BlockConflictResolvedError');
+		expect(err.message).toContain('block-1');
+	});
+});