fix(auth): use aff_code field name in registration payload (#4945) (#4965)

The new UI's sign-up form sent the invite code under key `aff`, but
the backend `Register` controller binds it to `User.AffCode` whose
JSON tag is `aff_code` (see model/user.go). Result: every invited
sign-up landed with `inviter_id = 0`, breaking the affiliate flow.

Rename only the request payload field so it matches the backend
contract. URL query parameter (`/sign-up?aff=...`), localStorage key
and OAuth state continue to use `aff` and are unchanged.

Closes #4945

web/default/src/features/auth/sign-up/components/sign-up-form.tsx

@@ -155,7 +155,7 @@ export function SignUpForm({
         password: data.password,
         email: data.email || undefined,
         verification_code: verificationCode || undefined,
-        aff: getAffiliateCode(),
+        aff_code: getAffiliateCode(),
         turnstile: turnstileToken,
       })
 

web/default/src/features/auth/types.ts

@@ -37,7 +37,7 @@ export interface RegisterPayload {
   password: string
   email?: string
   verification_code?: string
-  aff?: string
+  aff_code?: string
   turnstile?: string
 }