refactor: replace pdfjs-dist with native browser iframe for PDF preview

Remove pdfjs-dist dependency and simplify PdfPreview component to use
a native browser iframe, reducing bundle size and complexity.

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

frontend/package.json

@@ -10,7 +10,6 @@
   },
   "dependencies": {
     "chart.js": "^4.4.1",
-    "pdfjs-dist": "^4.0.379",
     "vue": "^3.4.21",
     "vue-chartjs": "^5.3.0",
     "vue-router": "^4.6.4"