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/e2e/fixtures/minimal.pdf

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+0000000009 00000 n 
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+0000000105 00000 n 
+trailer
+<</Size 4/Root 1 0 R>>
+startxref
+170
+%%EOF