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

start.sh

@@ -0,0 +1,23 @@
+#!/bin/bash
+
+# DigiServer v2 - Simple Start Script
+# Starts the application with proper configuration
+
+set -e
+
+cd /srv/digiserver-v2
+
+# Activate virtual environment
+source venv/bin/activate
+
+# Set environment variables
+export FLASK_APP=app.app:create_app
+export FLASK_ENV=development
+
+# Start Flask server
+echo "Starting DigiServer v2..."
+echo "Access at: http://localhost:5000"
+echo "Login: admin / admin123"
+echo ""
+
+flask run --host=0.0.0.0 --port=5000