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/watch.html

@@ -5,12 +5,12 @@
 {% block content %}
 <div class="watch-page">
     <div class="video-player-container">
-        {% if video.download_status == 'completed' and video.download_path %}
+        {% if video.download_status.value == 'completed' and video.download_path %}
             <video controls class="video-player">
                 <source src="/api/video/stream/{{ video.id }}" type="video/mp4">
                 Your browser does not support the video tag.
             </video>
-        {% elif video.download_status == 'downloading' %}
+        {% elif video.download_status.value == 'downloading' %}
             <div class="video-placeholder">
                 <div class="placeholder-content">
                     <h3>Video is downloading...</h3>
@@ -18,7 +18,7 @@
                     <button class="btn btn-secondary" onclick="location.reload()">Refresh</button>
                 </div>
             </div>
-        {% elif video.download_status == 'failed' %}
+        {% elif video.download_status.value == 'failed' %}
             <div class="video-placeholder error">
                 <div class="placeholder-content">
                     <h3>Download failed</h3>
@@ -49,7 +49,7 @@
             </div>
             <div class="video-stats">
                 <span class="publish-date">Published: {{ video.published_at.strftime('%B %d, %Y') }}</span>
-                {% if video.download_status == 'completed' and video.download_completed_at %}
+                {% if video.download_status.value == 'completed' and video.download_completed_at %}
                     <span class="download-date">Downloaded: {{ video.download_completed_at.strftime('%B %d, %Y') }}</span>
                 {% endif %}
             </div>
@@ -64,7 +64,7 @@
 
         <div class="video-links">
             <a href="{{ video.video_url }}" target="_blank" class="btn btn-link">Watch on YouTube</a>
-            {% if video.download_status == 'completed' and video.download_path %}
+            {% if video.download_status.value == 'completed' and video.download_path %}
                 <a href="/api/video/stream/{{ video.id }}?download=1" class="btn btn-secondary" download>Download MP4</a>
             {% endif %}
         </div>
@@ -103,7 +103,7 @@
     }
 
     // Auto-refresh if video is downloading
-    {% if video.download_status == 'downloading' %}
+    {% if video.download_status.value == 'downloading' %}
         setTimeout(() => location.reload(), 10000); // Refresh every 10 seconds
     {% endif %}
 </script>