Add dark mode support and replace group assignment with playlist assignment

- Added comprehensive dark mode styling to all pages:
  * Dashboard (workflow guide, secondary text, log items)
  * Admin panel with user management system
  * Content/playlist management page
  * Upload media page
  * Add player page

- Implemented user management system:
  * Create/edit/delete users
  * Two role types (user/admin)
  * Reset password functionality
  * Role-based permissions

- Replaced group assignment with playlist assignment:
  * Players now directly assigned to playlists
  * Updated add player form and backend
  * Removed group selection from player creation

- Fixed bugs:
  * Updated instance_path configuration for SQLite
  * Fixed import path in app factory
  * Updated dependencies (Pillow 11.0.0, removed gevent)

- Added start.sh script for easy development server launch

requirements.txt

@@ -24,7 +24,7 @@ python-dateutil==2.9.0
 
 # File Processing
 pdf2image==1.17.0
-Pillow==10.0.1
+Pillow==11.0.0
 ffmpeg-python==0.2.0
 python-magic==0.4.27
 
@@ -34,8 +34,8 @@ Flask-Talisman==1.1.0
 Flask-Cors==4.0.0
 
 # Production Server
-gunicorn==20.1.0
-gevent==23.9.1
+gunicorn==23.0.0
+# gevent==23.9.1  # Commented out - not compatible with Python 3.13 yet
 
 # Monitoring
 psutil==6.1.0