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/youtube_downloader.py

@@ -1,14 +1,19 @@
 import os
+import json
+import logging
 from abc import ABC
-from typing import Union, Optional
+from typing import Union, Optional, List
 
 import yt_dlp
 
 from app.downloaders.base import Downloader, DownloadQuality
 from app.models.notes_model import AudioDownloadResult
+from app.models.transcriber_model import TranscriptResult, TranscriptSegment
 from app.utils.path_helper import get_data_dir
 from app.utils.url_parser import extract_video_id
 
+logger = logging.getLogger(__name__)
+
 
 class YoutubeDownloader(Downloader, ABC):
     def __init__(self):
@@ -92,3 +97,119 @@ class YoutubeDownloader(Downloader, ABC):
             raise FileNotFoundError(f"视频文件未找到: {video_path}")
 
         return video_path
+
+    def download_subtitles(self, video_url: str, output_dir: str = None,
+                           langs: List[str] = None) -> Optional[TranscriptResult]:
+        """
+        尝试获取YouTube视频字幕（优先人工字幕，其次自动生成）
+
+        :param video_url: 视频链接
+        :param output_dir: 输出路径
+        :param langs: 优先语言列表
+        :return: TranscriptResult 或 None
+        """
+        if output_dir is None:
+            output_dir = get_data_dir()
+        if not output_dir:
+            output_dir = self.cache_data
+        os.makedirs(output_dir, exist_ok=True)
+
+        if langs is None:
+            langs = ['zh-Hans', 'zh', 'zh-CN', 'zh-TW', 'en', 'en-US']
+
+        video_id = extract_video_id(video_url, "youtube")
+
+        ydl_opts = {
+            'writesubtitles': True,
+            'writeautomaticsub': True,
+            'subtitleslangs': langs,
+            'subtitlesformat': 'json3',
+            'skip_download': True,
+            'outtmpl': os.path.join(output_dir, f'{video_id}.%(ext)s'),
+            'quiet': True,
+        }
+
+        try:
+            with yt_dlp.YoutubeDL(ydl_opts) as ydl:
+                info = ydl.extract_info(video_url, download=True)
+
+                # 查找下载的字幕文件
+                subtitles = info.get('requested_subtitles') or {}
+                if not subtitles:
+                    logger.info(f"YouTube视频 {video_id} 没有可用字幕")
+                    return None
+
+                # 按优先级查找字幕文件
+                subtitle_file = None
+                detected_lang = None
+                for lang in langs:
+                    if lang in subtitles:
+                        subtitle_file = os.path.join(output_dir, f"{video_id}.{lang}.json3")
+                        detected_lang = lang
+                        break
+
+                # 如果按优先级没找到，取第一个可用的
+                if not subtitle_file:
+                    for lang, sub_info in subtitles.items():
+                        subtitle_file = os.path.join(output_dir, f"{video_id}.{lang}.json3")
+                        detected_lang = lang
+                        break
+
+                if not subtitle_file or not os.path.exists(subtitle_file):
+                    logger.info(f"字幕文件不存在: {subtitle_file}")
+                    return None
+
+                # 解析字幕文件
+                return self._parse_json3_subtitle(subtitle_file, detected_lang)
+
+        except Exception as e:
+            logger.warning(f"获取YouTube字幕失败: {e}")
+            return None
+
+    def _parse_json3_subtitle(self, subtitle_file: str, language: str) -> Optional[TranscriptResult]:
+        """
+        解析 json3 格式字幕文件
+
+        :param subtitle_file: 字幕文件路径
+        :param language: 语言代码
+        :return: TranscriptResult
+        """
+        try:
+            with open(subtitle_file, 'r', encoding='utf-8') as f:
+                data = json.load(f)
+
+            segments = []
+            events = data.get('events', [])
+
+            for event in events:
+                # json3 格式中时间单位是毫秒
+                start_ms = event.get('tStartMs', 0)
+                duration_ms = event.get('dDurationMs', 0)
+
+                # 提取文本
+                segs = event.get('segs', [])
+                text = ''.join(seg.get('utf8', '') for seg in segs).strip()
+
+                if text:  # 只添加非空文本
+                    segments.append(TranscriptSegment(
+                        start=start_ms / 1000.0,
+                        end=(start_ms + duration_ms) / 1000.0,
+                        text=text
+                    ))
+
+            if not segments:
+                return None
+
+            full_text = ' '.join(seg.text for seg in segments)
+
+            logger.info(f"成功解析YouTube字幕，共 {len(segments)} 段")
+            return TranscriptResult(
+                language=language,
+                full_text=full_text,
+                segments=segments,
+                raw={'source': 'youtube_subtitle', 'file': subtitle_file}
+            )
+
+        except Exception as e:
+            logger.warning(f"解析字幕文件失败: {e}")
+            return None