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/routes/documents/[id]/+page.svelte

@@ -3,6 +3,7 @@ import { m } from '$lib/paraglide/messages.js';
 import { formatDate } from '$lib/utils/date';
 import { diffWords } from 'diff';
 import ExpandableText from '$lib/components/ExpandableText.svelte';
+import PdfViewer from '$lib/components/PdfViewer.svelte';
 
 let { data } = $props();
 
@@ -873,16 +874,12 @@ function versionLabel(v: VersionSummary, index: number): string {
 					</div>
 					<p class="font-sans text-sm tracking-wide uppercase">{m.doc_no_scan()}</p>
 				</div>
-			{:else if fileUrl && doc.originalFilename.toLowerCase().endsWith('.pdf')}
-				<iframe
-					src="{fileUrl}#zoom=page-width"
-					title={m.doc_preview_iframe_title()}
-					class="h-full w-full border-none bg-white"
-				></iframe>
+			{:else if fileUrl && doc.contentType?.startsWith('application/pdf')}
+				<PdfViewer url={fileUrl} />
 			{:else if fileUrl}
 				<div class="flex h-full w-full items-center justify-center overflow-auto p-8">
 					<img
-						src="{fileUrl}#zoom=page-width"
+						src={fileUrl}
 						alt={m.doc_image_alt()}
 						class="max-h-full max-w-full object-contain shadow-2xl"
 					/>