feat(admin): allow setting owner when creating an account

Adds optional owner_email field to the Create Account modal. Superadmin
can specify an existing user's email to assign as account owner at
creation time. Backend 404s with a clear message if the email is unknown.
Error detail now surfaces to the toast instead of a generic message.

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

backend/app/schemas/admin.py

@@ -126,6 +126,7 @@ class AdminAccountDetailResponse(AdminAccountListItem):
 class AdminAccountCreate(BaseModel):
     name: str = Field(..., min_length=1, max_length=255)
     plan: Literal["free", "pro", "team"] = "free"
+    owner_email: Optional[str] = Field(None, description="Email of an existing user to set as owner")
 
 
 class AdminAccountUpdate(BaseModel):