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

app/blueprints/admin.py

@@ -267,6 +267,55 @@ def clear_logs():
     return redirect(url_for('admin.admin_panel'))
 
 
+@admin_bp.route('/users')
+@login_required
+@admin_required
+def user_management():
+    """Display user management page."""
+    try:
+        users = User.query.order_by(User.created_at.desc()).all()
+        return render_template('admin/user_management.html', users=users)
+    except Exception as e:
+        log_action('error', f'Error loading user management: {str(e)}')
+        flash('Error loading user management page.', 'danger')
+        return redirect(url_for('admin.admin_panel'))
+
+
+@admin_bp.route('/user/<int:user_id>/password', methods=['POST'])
+@login_required
+@admin_required
+def reset_user_password(user_id: int):
+    """Reset user password."""
+    try:
+        user = User.query.get_or_404(user_id)
+        new_password = request.form.get('password', '').strip()
+        
+        # Validation
+        if not new_password or len(new_password) < 6:
+            flash('Password must be at least 6 characters long.', 'warning')
+            return redirect(url_for('admin.user_management'))
+        
+        # Prevent changing own password through this route
+        if user.id == current_user.id:
+            flash('Use the change password option to update your own password.', 'warning')
+            return redirect(url_for('admin.user_management'))
+        
+        # Update password
+        hashed_password = bcrypt.generate_password_hash(new_password).decode('utf-8')
+        user.password = hashed_password
+        db.session.commit()
+        
+        log_action('info', f'Password reset for user {user.username} by admin {current_user.username}')
+        flash(f'Password reset successfully for user "{user.username}".', 'success')
+        
+    except Exception as e:
+        db.session.rollback()
+        log_action('error', f'Error resetting password: {str(e)}')
+        flash('Error resetting password. Please try again.', 'danger')
+    
+    return redirect(url_for('admin.user_management'))
+
+
 @admin_bp.route('/system/info')
 @login_required
 @admin_required