Add task list page for Video Learning sessions

This commit adds a dedicated task list page to view and manage all video
learning sessions, solving the issue where users couldn't find their
background tasks after navigating away.

Features:
- New sessions.html page with card-based layout for all sessions
- Real-time polling for session status updates (every 3 seconds)
- Session control buttons (pause/resume/stop/delete)
- localStorage integration for session persistence across page refreshes
- Navigation links added to main page and video learning page
- Empty state UI when no sessions exist

New files:
- dashboard/static/sessions.html - Task list page
- dashboard/static/js/sessions.js - Sessions module with API calls
- dashboard/static/css/sessions.css - Styling for sessions page

Modified files:
- dashboard/api/video_learning.py - Added /sessions/list endpoint
- dashboard/static/index.html - Added "任务列表" button
- dashboard/static/video-learning.html - Added "任务列表" button and localStorage

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

dashboard/api/video_learning.py

@@ -351,6 +351,29 @@ async def list_sessions() -> List[str]:
     return list(_active_sessions.keys())
 
 
+@router.get("/sessions/list")
+async def list_all_sessions() -> List[Dict[str, Any]]:
+    """获取所有会话的详细信息（包含进度、状态等）"""
+    result = []
+    for session_id, agent in _active_sessions.items():
+        try:
+            progress = agent.get_session_progress()
+            result.append({
+                "session_id": session_id,
+                "platform": progress.get("platform", ""),
+                "target_count": progress.get("target_count", 0),
+                "watched_count": progress.get("watched_count", 0),
+                "progress_percent": progress.get("progress_percent", 0),
+                "is_active": progress.get("is_active", False),
+                "is_paused": progress.get("is_paused", False),
+                "total_duration": progress.get("total_duration", 0),
+            })
+        except Exception as e:
+            # 跳过出错的会话
+            continue
+    return result
+
+
 @router.delete("/sessions/{session_id}", response_model=Dict[str, str])
 async def delete_session(session_id: str) -> Dict[str, str]:
     """Delete a session and clean up device mapping."""