feat: multi-module improvements across merchant, store, i18n, and customer systems

- Fix platform-grouped merchant sidebar menu with core items at root level
- Add merchant store management (detail page, create store, team page)
- Fix store settings 500 error by removing dead stripe/API tab
- Move onboarding translations to module-owned locale files
- Fix onboarding banner i18n with server-side rendering + context inheritance
- Refactor login language selectors to use languageSelector() function (LANG-002)
- Move HTTPException handling to global exception handler in merchant routes (API-003)
- Add language selector to all login pages and portal headers
- Fix customer module: drop order stats from customer model, add to orders module
- Fix admin menu config visibility for super admin platform context
- Fix storefront auth and layout issues
- Add missing i18n translations for onboarding steps (en/fr/de/lb)

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

app/modules/tenancy/schemas/__init__.py

@@ -80,6 +80,9 @@ from app.modules.tenancy.schemas.merchant import (
     MerchantPortalProfileUpdate,
     MerchantPortalStoreListResponse,
     MerchantResponse,
+    MerchantStoreCreate,
+    MerchantStoreDetailResponse,
+    MerchantStoreUpdate,
     MerchantSummary,
     MerchantTransferOwnership,
     MerchantTransferOwnershipResponse,
@@ -163,6 +166,9 @@ __all__ = [
     "MerchantPortalProfileUpdate",
     "MerchantPortalStoreListResponse",
     "MerchantResponse",
+    "MerchantStoreCreate",
+    "MerchantStoreDetailResponse",
+    "MerchantStoreUpdate",
     "MerchantSummary",
     "MerchantTransferOwnership",
     "MerchantTransferOwnershipResponse",

app/modules/tenancy/schemas/merchant.py

@@ -261,3 +261,73 @@ class MerchantPortalStoreListResponse(BaseModel):
     total: int
     skip: int
     limit: int
+    can_create_store: bool = True
+
+
+class MerchantStoreCreate(BaseModel):
+    """Store creation from the merchant portal.
+    Subset of admin StoreCreate — excludes admin-only fields."""
+
+    name: str = Field(..., min_length=2, max_length=255, description="Store name")
+    store_code: str = Field(
+        ..., min_length=2, max_length=50, description="Unique store code"
+    )
+    subdomain: str = Field(
+        ..., min_length=2, max_length=100, description="Store subdomain"
+    )
+    description: str | None = Field(None, description="Store description")
+    platform_ids: list[int] = Field(
+        default_factory=list, description="Platform IDs to assign store to"
+    )
+
+    @field_validator("subdomain")
+    @classmethod
+    def validate_subdomain(cls, v):
+        """Validate subdomain format."""
+        import re
+
+        v = v.lower().strip()
+        if not re.match(r"^[a-z0-9][a-z0-9-]*[a-z0-9]$", v) and len(v) > 1:
+            raise ValueError(
+                "Subdomain must contain only lowercase letters, numbers, and hyphens"
+            )
+        return v
+
+    @field_validator("store_code")
+    @classmethod
+    def normalize_store_code(cls, v):
+        """Normalize store code to uppercase."""
+        return v.upper().strip()
+
+
+class MerchantStoreDetailResponse(BaseModel):
+    """Store detail for the merchant portal."""
+
+    id: int
+    store_code: str
+    subdomain: str
+    name: str
+    description: str | None = None
+    is_active: bool
+    is_verified: bool
+    contact_email: str | None = None
+    contact_phone: str | None = None
+    website: str | None = None
+    business_address: str | None = None
+    tax_number: str | None = None
+    default_language: str | None = None
+    created_at: str | None = None
+    platforms: list[dict] = Field(default_factory=list)
+
+
+class MerchantStoreUpdate(BaseModel):
+    """Store update from the merchant portal.
+    Only merchant-allowed fields."""
+
+    name: str | None = Field(None, min_length=2, max_length=255)
+    description: str | None = None
+    contact_email: EmailStr | None = None
+    contact_phone: str | None = None
+    website: str | None = None
+    business_address: str | None = None
+    tax_number: str | None = None