feat: loyalty feature provider, admin data fixes, storefront mobile menu

- Add LoyaltyFeatureProvider with 11 BINARY/MERCHANT features for billing
  feature gating, wired into loyalty module definition
- Fix subscription-tiers admin page showing 0 features by populating
  feature_codes from tier relationship in all admin tier endpoints
- Fix merchants admin page showing 0 stores and N/A owner by adding
  store_count and owner_email to MerchantResponse and eager-loading owner
- Add "no tiers" warning with link in subscription creation modal when
  platform has no configured tiers
- Add missing mobile menu panel to storefront base template so hamburger
  toggle actually shows navigation links

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

app/modules/billing/routes/api/admin.py

@@ -67,6 +67,7 @@ def list_subscription_tiers(
     for t in tiers:
         resp = SubscriptionTierResponse.model_validate(t)
         resp.platform_name = platforms_map.get(t.platform_id) if t.platform_id else None
+        resp.feature_codes = sorted(t.get_feature_codes())
         tiers_response.append(resp)
 
     return SubscriptionTierListResponse(
@@ -83,7 +84,9 @@ def get_subscription_tier(
 ):
     """Get a specific subscription tier by code."""
     tier = admin_subscription_service.get_tier_by_code(db, tier_code)
-    return SubscriptionTierResponse.model_validate(tier)
+    resp = SubscriptionTierResponse.model_validate(tier)
+    resp.feature_codes = sorted(tier.get_feature_codes())
+    return resp
 
 
 @admin_router.post("/tiers", response_model=SubscriptionTierResponse, status_code=201)
@@ -96,7 +99,9 @@ def create_subscription_tier(
     tier = admin_subscription_service.create_tier(db, tier_data.model_dump())
     db.commit()
     db.refresh(tier)
-    return SubscriptionTierResponse.model_validate(tier)
+    resp = SubscriptionTierResponse.model_validate(tier)
+    resp.feature_codes = sorted(tier.get_feature_codes())
+    return resp
 
 
 @admin_router.patch("/tiers/{tier_code}", response_model=SubscriptionTierResponse)
@@ -111,7 +116,9 @@ def update_subscription_tier(
     tier = admin_subscription_service.update_tier(db, tier_code, update_data)
     db.commit()
     db.refresh(tier)
-    return SubscriptionTierResponse.model_validate(tier)
+    resp = SubscriptionTierResponse.model_validate(tier)
+    resp.feature_codes = sorted(tier.get_feature_codes())
+    return resp
 
 
 @admin_router.delete("/tiers/{tier_code}", status_code=204)