refactor(astrolabe): extract PDF viewer to dedicated component

Replace fragile 20-iteration retry loop with proper Vue lifecycle management.

Changes:
- Create PDFViewer.vue component (~200 lines) with:
  - Proper mounted/beforeUnmount lifecycle hooks
  - Loading/error/content states
  - Page navigation controls
  - PDF document cleanup
  - User-friendly error messages
- Simplify App.vue by removing ~120 lines:
  - Remove loadPdf() and renderPdfPage() methods
  - Remove manual DOM polling with $nextTick() loops
  - Replace PDF template with <PDFViewer> component
- Add pdfjs-dist@4.0.379 dependency

Result: Canvas found on first attempt instead of requiring 20 retries.

Follows patterns from Nextcloud's production files_pdfviewer app and
2025 Vue.js + PDF.js best practices.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

third_party/astroglobe/package.json

@@ -23,6 +23,7 @@
     "@nextcloud/router": "^3.0.1",
     "@nextcloud/vue": "^8.29.2",
     "plotly.js-dist-min": "^2.35.3",
+    "pdfjs-dist": "^4.0.379",
     "vue": "^2.7.16",
     "vue-material-design-icons": "^5.3.1"
   },