Add Video Learning Agent for short video platforms

Features:
- VideoLearningAgent for automated video watching on Douyin/Kuaishou/TikTok
- Web dashboard UI for video learning sessions
- Real-time progress tracking with screenshot capture
- App detection using get_current_app() for accurate recording
- Session management with pause/resume/stop controls

Technical improvements:
- Simplified video detection logic using direct app detection
- Full base64 hash for sensitive screenshot change detection
- Immediate stop when target video count is reached
- Fixed circular import issues with ModelConfig

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

.env.example

@@ -108,3 +108,16 @@ SCREENSHOT_THROTTLE_MS=500
 
 # Maximum task history to keep / 保留的最大任务历史数
 MAX_TASK_HISTORY=100
+
+# ============================================================================
+# Video Learning Configuration / 视频学习配置
+# ============================================================================
+
+# Output directory for video learning data / 视频学习数据输出目录
+VIDEO_LEARNING_OUTPUT_DIR=./video_learning_data
+
+# Model parameters for video learning / 视频学习模型参数
+PHONE_AGENT_MAX_TOKENS=3000
+PHONE_AGENT_TEMPERATURE=0.0
+PHONE_AGENT_TOP_P=0.85
+PHONE_AGENT_FREQUENCY_PENALTY=0.2