Add feed deletion and single video addition features

- Implemented DELETE /api/channels/<id> to remove channels and cleanup downloaded files
- Added delete button to channels page with confirmation dialog
- Added functionality to add single videos via URL
- Updated navigation menu

templates/base.html

@@ -16,6 +16,7 @@
                     <li><a href="/channels" class="{% if request.path == '/channels' %}active{% endif %}">Channels</a></li>
                     <li><a href="/downloads" class="{% if request.path == '/downloads' %}active{% endif %}">Downloads</a></li>
                     <li><a href="/add-channel" class="{% if request.path == '/add-channel' %}active{% endif %}">Add Channel</a></li>
+                    <li><a href="/add-video" class="{% if request.path == '/add-video' %}active{% endif %}">Add Video</a></li>
                     <li class="nav-user">
                         <span>{{ current_user.username }}</span>
                         <a href="{{ url_for('logout') }}">Logout</a>