feat(frontend): replace iframe with PDF.js viewer (#39)

- Install pdfjs-dist v5 and add optimizeDeps pre-bundle config
- New PdfViewer.svelte component: renders each page on a <canvas> with
  correct device-pixel-ratio scaling, overlays a text layer (enables
  text selection; foundation for annotations in #40), prev/next
  navigation, zoom controls, and lazy page rendering (only current ±1
  pre-fetched — avoids freezing on multi-page documents)
- Replace the <iframe> in documents/[id]/+page.svelte with PdfViewer;
  image attachments continue to use <img>; detection now uses
  doc.contentType instead of filename extension
- Unit tests for navigation controls and page counter (pdfjs mocked)
- E2E tests: PDF renders as canvas (not iframe), nav controls visible,
  image fallback stays as <img>; minimal.pdf fixture for upload tests

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

frontend/src/lib/components/PdfViewer.svelte.spec.ts

@@ -0,0 +1,53 @@
+import { vi, describe, it, expect, afterEach } from 'vitest';
+import { cleanup, render } from 'vitest-browser-svelte';
+import { page } from 'vitest/browser';
+
+// pdfjs-dist is a rendering dependency — we mock it so unit tests don't need
+// a real browser PDF engine. The interesting behaviour under test here is the
+// component's own UI logic (controls, page counter), not pdfjs internals.
+vi.mock('pdfjs-dist', () => {
+	function TextLayerMock() {}
+	TextLayerMock.prototype.render = () => Promise.resolve();
+	TextLayerMock.prototype.cancel = () => {};
+
+	return {
+		GlobalWorkerOptions: { workerSrc: '' },
+		getDocument: vi.fn().mockReturnValue({
+			promise: Promise.resolve({
+				numPages: 2,
+				getPage: vi.fn().mockResolvedValue({
+					getViewport: vi.fn().mockReturnValue({ width: 595, height: 842 }),
+					render: vi.fn().mockReturnValue({ promise: Promise.resolve() }),
+					streamTextContent: vi.fn().mockReturnValue(new ReadableStream())
+				})
+			})
+		}),
+		TextLayer: TextLayerMock
+	};
+});
+
+vi.mock('pdfjs-dist/build/pdf.worker.min.mjs?url', () => ({ default: '' }));
+
+import PdfViewer from './PdfViewer.svelte';
+
+afterEach(cleanup);
+
+describe('PdfViewer', () => {
+	it('shows previous and next page navigation buttons', async () => {
+		render(PdfViewer, { url: '/api/documents/test-id/file' });
+		await expect.element(page.getByRole('button', { name: /zurück/i })).toBeInTheDocument();
+		await expect.element(page.getByRole('button', { name: /weiter/i })).toBeInTheDocument();
+	});
+
+	it('shows zoom controls', async () => {
+		render(PdfViewer, { url: '/api/documents/test-id/file' });
+		await expect.element(page.getByRole('button', { name: /vergrößern/i })).toBeInTheDocument();
+		await expect.element(page.getByRole('button', { name: /verkleinern/i })).toBeInTheDocument();
+	});
+
+	it('displays the page counter once the PDF has loaded', async () => {
+		render(PdfViewer, { url: '/api/documents/test-id/file' });
+		// Mock resolves synchronously, so "1 / 2" should appear quickly
+		await expect.element(page.getByText(/1\s*\/\s*2/)).toBeInTheDocument();
+	});
+});