feat: dynamic merchant sidebar with module-driven menus

Replace the hardcoded merchant sidebar with a dynamic menu system driven
by module definitions, matching the existing admin frontend pattern.
Modules declare FrontendType.MERCHANT menus in their definition.py, and
a new API endpoint unions enabled modules across all platforms the
merchant is subscribed to — so loyalty only appears when enabled.

- Add MERCHANT menu definitions to core, billing, tenancy, loyalty modules
- Extend MenuDiscoveryService with enabled_module_codes parameter
- Create GET /merchants/core/menu/render/merchant endpoint
- Update merchant Alpine.js with loadMenuConfig() and dynamic section state
- Replace hardcoded sidebar.html with x-for rendering + loading skeleton + fallback
- Add 36 unit and integration tests for menu discovery, service, and endpoint

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/modules/core/definition.py

@@ -74,6 +74,9 @@ core_module = ModuleDefinition(
             "settings",
             "email-templates",
         ],
+        FrontendType.MERCHANT: [
+            "dashboard",
+        ],
     },
     # New module-driven menu definitions
     menus={
@@ -121,6 +124,25 @@ core_module = ModuleDefinition(
                 ],
             ),
         ],
+        FrontendType.MERCHANT: [
+            MenuSectionDefinition(
+                id="main",
+                label_key=None,
+                icon=None,
+                order=0,
+                is_collapsible=False,
+                items=[
+                    MenuItemDefinition(
+                        id="dashboard",
+                        label_key="core.menu.dashboard",
+                        icon="home",
+                        route="/merchants/dashboard",
+                        order=10,
+                        is_mandatory=True,
+                    ),
+                ],
+            ),
+        ],
         FrontendType.STORE: [
             MenuSectionDefinition(
                 id="main",