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/downloads.html

@@ -0,0 +1,79 @@
+{% extends "base.html" %}
+
+{% block title %}Downloads - YottoB{% endblock %}
+
+{% block content %}
+<div class="downloads-page">
+    <div class="page-header">
+        <h2>Download Jobs</h2>
+        <p>View and manage all video downloads</p>
+    </div>
+
+    {% if videos %}
+        <div class="downloads-list">
+            {% for video in videos %}
+            <div class="download-card">
+                <div class="download-thumbnail">
+                    {% if video.thumbnail_url %}
+                        <img src="{{ video.thumbnail_url }}" alt="{{ video.title }}">
+                    {% else %}
+                        <div class="thumbnail-placeholder">No thumbnail</div>
+                    {% endif %}
+                </div>
+
+                <div class="download-info">
+                    <h3 class="download-title">
+                        <a href="/watch/{{ video.id }}">{{ video.title }}</a>
+                    </h3>
+                    <p class="download-channel">{{ video.channel.title }}</p>
+                    <div class="download-meta">
+                        <span class="download-date">Published: {{ video.published_at.strftime('%Y-%m-%d') }}</span>
+                        {% if video.download_started_at %}
+                            <span class="download-date">Started: {{ video.download_started_at.strftime('%Y-%m-%d %H:%M') }}</span>
+                        {% endif %}
+                        {% if video.download_completed_at %}
+                            <span class="download-date">Completed: {{ video.download_completed_at.strftime('%Y-%m-%d %H:%M') }}</span>
+                        {% endif %}
+                    </div>
+                </div>
+
+                <div class="download-status-section">
+                    {% if video.download_status.value == 'completed' %}
+                        <span class="badge badge-success">Completed</span>
+                        {% if video.file_size %}
+                            <span class="download-size">{{ (video.file_size / 1024 / 1024) | round(2) }} MB</span>
+                        {% endif %}
+                    {% elif video.download_status.value == 'downloading' %}
+                        <span class="badge badge-info">Downloading...</span>
+                    {% elif video.download_status.value == 'failed' %}
+                        <span class="badge badge-error">Failed</span>
+                        {% if video.download_error %}
+                            <p class="download-error">{{ video.download_error }}</p>
+                        {% endif %}
+                    {% else %}
+                        <span class="badge badge-info">Pending</span>
+                    {% endif %}
+                </div>
+
+                <div class="download-actions">
+                    {% if video.download_status.value == 'pending' or video.download_status.value == 'failed' %}
+                        <form method="POST" action="/api/download/{{ video.id }}" style="display: inline;">
+                            <button type="submit" class="btn btn-primary">Download</button>
+                        </form>
+                    {% elif video.download_status.value == 'completed' %}
+                        <a href="/watch/{{ video.id }}" class="btn btn-primary">Watch</a>
+                        <a href="/api/video/stream/{{ video.id }}?download=1" class="btn btn-secondary">Download File</a>
+                    {% endif %}
+                </div>
+            </div>
+            {% endfor %}
+        </div>
+    {% else %}
+        <div class="empty-state">
+            <h3>No Videos Yet</h3>
+            <p>Subscribe to channels to see videos here.</p>
+            <a href="/add-channel" class="btn btn-primary">Add Channel</a>
+        </div>
+    {% endif %}
+</div>
+{% endblock %}