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/static/index.html

@@ -48,6 +48,15 @@
                     </svg>
                     Video Learning
                 </a>
+                <a href="/static/sessions.html" class="btn btn-secondary">
+                    <svg width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2">
+                        <rect x="3" y="3" width="7" height="7"></rect>
+                        <rect x="14" y="3" width="7" height="7"></rect>
+                        <rect x="14" y="14" width="7" height="7"></rect>
+                        <rect x="3" y="14" width="7" height="7"></rect>
+                    </svg>
+                    任务列表
+                </a>
                 <button @click="refreshDevices" class="btn btn-secondary" :disabled="refreshing">
                     <svg width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2" :class="{ spinning: refreshing }">
                         <polyline points="23 4 23 10 17 10"></polyline>