fix: make prefix required in TenantCreate schema and add frontend validation

prefix was optional with empty default, allowing accounts to be created
with broken account_code (no prefix). Root fix: enforce prefix at input
level rather than patching data after the fact.

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

backend/app/schemas/tenant.py

@@ -5,7 +5,7 @@ from pydantic import BaseModel, EmailStr
 
 class TenantBase(BaseModel):
     code: str
-    prefix: str = ""
+    prefix: str
     name: str
     name_eng: Optional[str] = None
     tax_id: Optional[str] = None