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>

app/config/__init__.py

@@ -0,0 +1,18 @@
+# app/config/__init__.py
+"""Configuration modules for the application."""
+
+from .menu_registry import (
+    ADMIN_MENU_REGISTRY,
+    VENDOR_MENU_REGISTRY,
+    AdminMenuItem,
+    VendorMenuItem,
+    get_all_menu_item_ids,
+)
+
+__all__ = [
+    "ADMIN_MENU_REGISTRY",
+    "VENDOR_MENU_REGISTRY",
+    "AdminMenuItem",
+    "VendorMenuItem",
+    "get_all_menu_item_ids",
+]