feat: add platform assignment to user and vendor creation forms

User create page:
- When role=admin, show super admin toggle
- If not super admin, show platform multi-select
- Admin users created via /api/v1/admin/admin-users endpoint
- Vendor users created via existing /admin/users endpoint

Vendor create page:
- Add platform selection section
- Vendors can be assigned to multiple platforms on creation
- Update VendorCreate schema to accept platform_ids
- Update AdminService.create_vendor() to create VendorPlatform records

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

models/schema/vendor.py

@@ -50,6 +50,11 @@ class VendorCreate(BaseModel):
     )
     description: str | None = Field(None, description="Vendor/brand description")
 
+    # Platform assignments (optional - vendor can be on multiple platforms)
+    platform_ids: list[int] | None = Field(
+        None, description="List of platform IDs to assign the vendor to"
+    )
+
     # Marketplace URLs (brand-specific multi-language support)
     letzshop_csv_url_fr: str | None = Field(None, description="French CSV URL")
     letzshop_csv_url_en: str | None = Field(None, description="English CSV URL")