Implement UI improvements and download management features

- Switch to light mode with black and white color scheme
- Simplify channel subscription to use channel ID only instead of RSS URL
- Add Downloads page to track all video download jobs
- Fix Flask-Login session management bug in user loader
- Always filter YouTube Shorts from feeds (case-insensitive)
- Fix download service video URL attribute error
- Fix watch page enum comparison for download status display

UI Changes:
- Update CSS to pure black/white/grayscale theme
- Remove colored text and buttons
- Use underlines for hover states instead of color changes
- Improve visual hierarchy with grayscale shades

Channel Subscription:
- Accept channel ID directly instead of full RSS URL
- Add validation for channel ID format (UC/UU prefix)
- Update help text and examples for easier onboarding

Downloads Page:
- New route at /downloads showing all video download jobs
- Display status, progress, and metadata for each download
- Sortable by status (downloading, pending, failed, completed)
- Actions to download, retry, or watch videos
- Responsive grid layout with thumbnails

Bug Fixes:
- Fix user loader to properly use database session context manager
- Fix download service accessing wrong attribute (link → video_url)
- Fix watch page template enum value comparisons
- Fix session detachment issues when accessing channel data

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

templates/add_channel.html

@@ -12,18 +12,17 @@
     <div class="form-container">
         <form method="POST" action="/add-channel" class="channel-form">
             <div class="form-group">
-                <label for="rss_url">YouTube Channel RSS URL</label>
+                <label for="channel_id">YouTube Channel ID</label>
                 <input
-                    type="url"
-                    id="rss_url"
-                    name="rss_url"
-                    placeholder="https://www.youtube.com/feeds/videos.xml?channel_id=..."
+                    type="text"
+                    id="channel_id"
+                    name="channel_id"
+                    placeholder="UC_x5XG1OV2P6uZZ5FSM9Ttw"
                     required
                     class="form-input"
                 >
                 <small class="form-help">
-                    Enter the RSS feed URL for the YouTube channel.
-                    Format: https://www.youtube.com/feeds/videos.xml?channel_id=CHANNEL_ID
+                    Enter the YouTube channel ID (starts with UC or UU).
                 </small>
             </div>
 
@@ -35,23 +34,24 @@
     </div>
 
     <div class="help-section">
-        <h3>How to find a channel's RSS URL</h3>
+        <h3>How to find a channel ID</h3>
         <ol>
             <li>Go to the YouTube channel page</li>
-            <li>Look at the URL in your browser - it will contain the channel ID</li>
-            <li>The channel URL format is usually:
+            <li>Look at the URL in your browser</li>
+            <li>If the URL is <code>youtube.com/channel/CHANNEL_ID</code>, copy the CHANNEL_ID part</li>
+            <li>If the URL is <code>youtube.com/@username</code>, you'll need to:
                 <ul>
-                    <li><code>youtube.com/channel/CHANNEL_ID</code> or</li>
-                    <li><code>youtube.com/@username</code> (you'll need to find the channel ID from the page source)</li>
+                    <li>Right-click the page and select "View Page Source"</li>
+                    <li>Search for "channelId" or "browse_id"</li>
+                    <li>Copy the ID that starts with "UC"</li>
                 </ul>
             </li>
-            <li>Use the format: <code>https://www.youtube.com/feeds/videos.xml?channel_id=CHANNEL_ID</code></li>
         </ol>
 
         <h4>Example</h4>
         <p>
-            For channel: <code>https://www.youtube.com/channel/UC_x5XG1OV2P6uZZ5FSM9Ttw</code><br>
-            RSS URL: <code>https://www.youtube.com/feeds/videos.xml?channel_id=UC_x5XG1OV2P6uZZ5FSM9Ttw</code>
+            For channel URL: <code>https://www.youtube.com/channel/UC_x5XG1OV2P6uZZ5FSM9Ttw</code><br>
+            Channel ID: <code>UC_x5XG1OV2P6uZZ5FSM9Ttw</code>
         </p>
     </div>
 </div>