feat: add admin menu configuration and sidebar improvements

- Add AdminMenuConfig model for per-platform menu customization
- Add menu registry for centralized menu configuration
- Add my-menu-config and platform-menu-config admin pages
- Update sidebar with improved layout and Alpine.js interactions
- Add FrontendType enum for admin/vendor menu separation
- Document self-contained module patterns in session note

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

models/database/user.py

@@ -73,6 +73,15 @@ class User(Base, TimestampMixin):
         cascade="all, delete-orphan",
     )
 
+    # Menu visibility configuration (for super admins only)
+    # Platform admins get menu config from their platform, not user-level
+    menu_configs = relationship(
+        "AdminMenuConfig",
+        foreign_keys="AdminMenuConfig.user_id",
+        back_populates="user",
+        cascade="all, delete-orphan",
+    )
+
     def __repr__(self):
         """String representation of the User object."""
         return f"<User(id={self.id}, username='{self.username}', email='{self.email}', role='{self.role}')>"