✨ Add comprehensive admin panel with user management

Features added:
- Admin authentication system with is_admin field
- Complete admin dashboard with user statistics
- User management (create, edit, delete, toggle admin)
- Protected admin routes with @admin_required decorator
- Security features (prevent self-deletion, last admin protection)
- Responsive admin UI with Bootstrap integration
- Database migration script for admin field
- Admin navigation link for authorized users

Technical improvements:
- Enhanced 3-column dashboard layout (tables | editor | saved queries)
- Removed plus button and made right sidebar more compact
- Admin user (admin/admin123) automatically created with admin privileges
- Full CRUD operations for user management
- Flash messages for user feedback
- Form validation and error handling

app/templates/base.html

@@ -23,8 +23,16 @@
             
             {% if current_user.is_authenticated %}
             <div class="navbar-nav ms-auto">
+                {% if current_user.is_admin %}
+                <a class="nav-link me-3" href="{{ url_for('admin.admin_dashboard') }}">
+                    <i class="fas fa-users-cog"></i> Admin
+                </a>
+                {% endif %}
                 <span class="navbar-text me-3">
                     <i class="fas fa-user"></i> {{ current_user.username }}
+                    {% if current_user.is_admin %}
+                    <span class="badge bg-danger ms-1">Admin</span>
+                    {% endif %}
                 </span>
                 <a class="nav-link" href="{{ url_for('auth.logout') }}">
                     <i class="fas fa-sign-out-alt"></i> Abmelden