feat: Add subtitle priority fetching and enhance mindmap export

## Subtitle Priority (Backend)
- Add download_subtitles() method to base downloader
- Implement Bilibili subtitle fetching with cookies support
- Implement YouTube subtitle fetching
- Support SRT and JSON3 format parsing
- Prioritize platform subtitles over Whisper transcription

## Mindmap Export Enhancements (Frontend)
- Add SVG vector export with proper viewBox handling
- Add XMind format export with Chinese character encoding fix
- Fix PNG/SVG export to capture full content by calling fit() before export
- Add JSZip dependency for XMind export

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

BillNote_frontend/package.json

@@ -32,6 +32,7 @@
     "clsx": "^2.1.1",
     "fuse.js": "^7.1.0",
     "github-markdown-css": "^5.8.1",
+    "jszip": "^3.10.1",
     "katex": "^0.16.22",
     "lottie-react": "^2.4.1",
     "lucide-react": "^0.487.0",