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/viewer/useFileLoader.svelte.test.ts

@@ -0,0 +1,98 @@
+import { describe, it, expect, vi, afterEach } from 'vitest';
+import { createFileLoader } from './useFileLoader.svelte';
+
+const FAKE_URL = 'blob:fake-url';
+
+function setupFetch(ok: boolean, body?: Blob) {
+	const blob = body ?? new Blob(['fake'], { type: 'application/pdf' });
+	vi.stubGlobal(
+		'fetch',
+		vi.fn().mockResolvedValue({
+			ok,
+			blob: vi.fn().mockResolvedValue(blob)
+		})
+	);
+}
+
+afterEach(() => {
+	vi.unstubAllGlobals();
+	vi.restoreAllMocks();
+});
+
+describe('createFileLoader', () => {
+	it('sets fileUrl after a successful fetch', async () => {
+		vi.stubGlobal('URL', {
+			createObjectURL: vi.fn().mockReturnValue(FAKE_URL),
+			revokeObjectURL: vi.fn()
+		});
+		setupFetch(true);
+
+		const loader = createFileLoader();
+		await loader.loadFile('/api/documents/1/file');
+
+		expect(loader.fileUrl).toBe(FAKE_URL);
+		expect(loader.isLoading).toBe(false);
+		expect(loader.fileError).toBe('');
+	});
+
+	it('sets fileError on a failed fetch (non-ok response)', async () => {
+		vi.stubGlobal('URL', {
+			createObjectURL: vi.fn(),
+			revokeObjectURL: vi.fn()
+		});
+		setupFetch(false);
+
+		const loader = createFileLoader();
+		await loader.loadFile('/api/documents/1/file');
+
+		expect(loader.fileUrl).toBe('');
+		expect(loader.fileError).not.toBe('');
+		expect(loader.isLoading).toBe(false);
+	});
+
+	it('revokes the previous URL before creating a new one', async () => {
+		const revokeObjectURL = vi.fn();
+		vi.stubGlobal('URL', {
+			createObjectURL: vi.fn().mockReturnValue(FAKE_URL),
+			revokeObjectURL
+		});
+		setupFetch(true);
+
+		const loader = createFileLoader();
+		await loader.loadFile('/api/documents/1/file');
+		// First load: no previous URL to revoke
+		expect(revokeObjectURL).not.toHaveBeenCalled();
+
+		await loader.loadFile('/api/documents/2/file');
+		// Second load: previous URL should be revoked
+		expect(revokeObjectURL).toHaveBeenCalledWith(FAKE_URL);
+	});
+
+	it('revokes the URL on destroy', async () => {
+		const revokeObjectURL = vi.fn();
+		vi.stubGlobal('URL', {
+			createObjectURL: vi.fn().mockReturnValue(FAKE_URL),
+			revokeObjectURL
+		});
+		setupFetch(true);
+
+		const loader = createFileLoader();
+		await loader.loadFile('/api/documents/1/file');
+		loader.destroy();
+
+		expect(revokeObjectURL).toHaveBeenCalledWith(FAKE_URL);
+	});
+
+	it('does not revoke when no URL has been set', () => {
+		const revokeObjectURL = vi.fn();
+		vi.stubGlobal('URL', {
+			createObjectURL: vi.fn(),
+			revokeObjectURL
+		});
+
+		const loader = createFileLoader();
+		loader.destroy();
+
+		expect(revokeObjectURL).not.toHaveBeenCalled();
+	});
+});