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>

backend/app/downloaders/base.py

@@ -5,6 +5,7 @@ from typing import Optional, Union
 
 from app.enmus.note_enums import DownloadQuality
 from app.models.notes_model import AudioDownloadResult
+from app.models.transcriber_model import TranscriptResult
 from os import getenv
 QUALITY_MAP = {
     "fast": "32",
@@ -36,3 +37,15 @@ class Downloader(ABC):
     def download_video(self, video_url: str,
                        output_dir: Union[str, None] = None) -> str:
         pass
+
+    def download_subtitles(self, video_url: str, output_dir: str = None,
+                           langs: list = None) -> Optional[TranscriptResult]:
+        '''
+        尝试获取平台字幕（人工字幕或自动生成字幕）
+
+        :param video_url: 视频链接
+        :param output_dir: 输出路径
+        :param langs: 优先语言列表，如 ['zh-Hans', 'zh', 'en']
+        :return: TranscriptResult 或 None（无字幕时）
+        '''
+        return None