refactor(signup): remove firstName/lastName fields (DSGVO data-minimization)

Felder wurden nirgendwo gelesen/angezeigt (nur in raw_user_meta_data
gespeichert ohne Verwendung). Inkonsistent mit OAuth-Flow der sie
gar nicht erfasst. Entfernt:
- 2 Inputs aus signup.tsx
- firstName/lastName aus signUp metadata-Typ + data
- 8 i18n-keys (de/en/fr/ar)
- DB-Cleanup via SQL für 5 existing User (raw_user_meta_data - 'first_name' - 'last_name')

Art. 5(1)c DSGVO: nur Daten verarbeiten die für Zweck notwendig sind.
Nickname allein reicht — Anonymität-Pattern (memory/feedback_anonymity_nickname.md).

apps/rebreak-native/app/(auth)/signup.tsx

@@ -53,8 +53,6 @@ export default function SignUpScreen() {
 
   const [email, setEmail] = useState('');
   const [password, setPassword] = useState('');
-  const [firstName, setFirstName] = useState('');
-  const [lastName, setLastName] = useState('');
   const [nickname, setNickname] = useState('');
   const [avatarId, setAvatarId] = useState('spider');
   const [termsAccepted, setTermsAccepted] = useState(false);
@@ -93,8 +91,6 @@ export default function SignUpScreen() {
     setSubmitting(true);
     const res = await signUp(email.trim(), password, {
       username: nickname.trim(),
-      firstName: firstName.trim() || undefined,
-      lastName: lastName.trim() || undefined,
       avatarId,
       avatarUrl: getAvatarUrl(avatarId),
     });
@@ -186,27 +182,6 @@ export default function SignUpScreen() {
             onChangeText={setPassword}
           />
 
-          <View className="flex-row gap-3 mb-3">
-            <TextInput
-              className="flex-1 bg-neutral-100 rounded-xl"
-              style={INPUT_STYLE}
-              placeholder={t('auth.firstName')}
-              placeholderTextColor="#a3a3a3"
-              autoComplete="given-name"
-              value={firstName}
-              onChangeText={setFirstName}
-            />
-            <TextInput
-              className="flex-1 bg-neutral-100 rounded-xl"
-              style={INPUT_STYLE}
-              placeholder={t('auth.lastName')}
-              placeholderTextColor="#a3a3a3"
-              autoComplete="family-name"
-              value={lastName}
-              onChangeText={setLastName}
-            />
-          </View>
-
           <TextInput
             className="bg-neutral-100 rounded-xl mb-6"
             style={INPUT_STYLE}

apps/rebreak-native/locales/ar.json

@@ -28,8 +28,6 @@
     "passwordRequired": "كلمة المرور * (8 أحرف على الأقل)",
     "passwordMin8": "يجب أن تتكون كلمة المرور من 8 أحرف على الأقل.",
     "newPassword": "كلمة مرور جديدة",
-    "firstName": "الاسم الأول",
-    "lastName": "اسم العائلة",
     "nickname": "اسم المستخدم",
     "nicknamePlaceholder": "اسم المستخدم * (مرئي للآخرين)",
     "noAccount": "ليس لديك حساب؟",

apps/rebreak-native/locales/de.json

@@ -28,8 +28,6 @@
     "passwordRequired": "Passwort * (min. 8 Zeichen)",
     "passwordMin8": "Passwort muss mindestens 8 Zeichen haben.",
     "newPassword": "Neues Passwort",
-    "firstName": "Vorname",
-    "lastName": "Nachname",
     "nickname": "Benutzername",
     "nicknamePlaceholder": "Benutzername * (sichtbar für andere)",
     "noAccount": "Noch kein Konto?",

apps/rebreak-native/locales/en.json

@@ -28,8 +28,6 @@
     "passwordRequired": "Password * (min. 8 characters)",
     "passwordMin8": "Password must be at least 8 characters.",
     "newPassword": "New password",
-    "firstName": "First name",
-    "lastName": "Last name",
     "nickname": "Username",
     "nicknamePlaceholder": "Username * (visible to others)",
     "noAccount": "No account yet?",

apps/rebreak-native/locales/fr.json

@@ -28,8 +28,6 @@
     "passwordRequired": "Mot de passe * (min. 8 caractères)",
     "passwordMin8": "Le mot de passe doit comporter au moins 8 caractères.",
     "newPassword": "Nouveau mot de passe",
-    "firstName": "Prénom",
-    "lastName": "Nom",
     "nickname": "Pseudo",
     "nicknamePlaceholder": "Pseudo * (visible par les autres)",
     "noAccount": "Pas encore de compte ?",

apps/rebreak-native/stores/auth.ts

@@ -47,7 +47,7 @@ type AuthState = {
   signUp: (
     email: string,
     password: string,
-    metadata: { username: string; firstName?: string; lastName?: string; avatarId: string; avatarUrl: string }
+    metadata: { username: string; avatarId: string; avatarUrl: string }
   ) => Promise<{ error?: string }>;
   signOut: () => Promise<void>;
   signInWithOAuth: (provider: 'google' | 'apple') => Promise<{ error?: string }>;
@@ -124,8 +124,6 @@ export const useAuthStore = create<AuthState>((set) => ({
       options: {
         data: {
           username: metadata.username,
-          first_name: metadata.firstName ?? '',
-          last_name: metadata.lastName ?? '',
           avatar_id: metadata.avatarId,
           avatar_url: metadata.avatarUrl,
           locale: currentLocale(),