feat: implement user authentication with login and register

- Add login screen with email/username support
- Add register screen with email validation
- Implement AuthStore with expo-secure-store (native) / localStorage (web)
- Add X-User-Id header authentication (simple auth without JWT)
- Rename displayName to userName across codebase
- Add findByUserName() to UserRepository
- Check for existing email AND username on registration
- Add AuthButton component with shadow effect
- Add logout button to Header
- Add hash-password.js utility script for manual password resets
- Update CORS to allow X-User-Id header

apps/client/src/app/login.tsx

@@ -1,31 +1,95 @@
+import { useState } from "react";
 import { View, Text, TextInput, Pressable } from "react-native";
+import { Link, router } from "expo-router";
 import BaseBackground from "../components/BaseBackground";
+import AuthButton from "../components/AuthButton";
+import { AuthService } from "../services";
+import currentTheme from "../Themes";
 
 const LoginScreen = () => {
-  // TODO: Email input field
-  // TODO: Password input field
-  // TODO: Login button -> AuthService.login()
-  // TODO: Link to RegisterScreen
-  // TODO: Error handling and display
-  // TODO: Navigate to Calendar on success
-  throw new Error("Not implemented");
+  const [identifier, setIdentifier] = useState("");
+  const [password, setPassword] = useState("");
+  const [error, setError] = useState<string | null>(null);
+  const [isLoading, setIsLoading] = useState(false);
+
+  const handleLogin = async () => {
+    setError(null);
+
+    if (!identifier || !password) {
+      setError("Bitte alle Felder ausfüllen");
+      return;
+    }
+
+    setIsLoading(true);
+    try {
+      await AuthService.login({ identifier, password });
+      router.replace("/(tabs)/chat");
+    } catch {
+      setError("Anmeldung fehlgeschlagen. Überprüfe deine Zugangsdaten.");
+    } finally {
+      setIsLoading(false);
+    }
+  };
 
   return (
     <BaseBackground>
-      <View className="flex-1 justify-center items-center p-4">
-        <Text className="text-2xl mb-8">Login</Text>
+      <View className="flex-1 justify-center items-center p-8">
+        <Text
+          className="text-3xl font-bold mb-8"
+          style={{ color: currentTheme.textPrimary }}
+        >
+          Anmelden
+        </Text>
+
+        {error && (
+          <Text className="mb-4 text-center" style={{ color: currentTheme.rejectButton }}>
+            {error}
+          </Text>
+        )}
+
         <TextInput
-          placeholder="Email"
-          className="w-full border rounded p-2 mb-4"
+          placeholder="E-Mail oder Benutzername"
+          placeholderTextColor={currentTheme.textMuted}
+          value={identifier}
+          onChangeText={setIdentifier}
+          autoCapitalize="none"
+          className="w-full rounded-lg p-4 mb-4"
+          style={{
+            backgroundColor: currentTheme.secondaryBg,
+            color: currentTheme.textPrimary,
+            borderWidth: 1,
+            borderColor: currentTheme.borderPrimary,
+          }}
         />
+
         <TextInput
-          placeholder="Password"
+          placeholder="Passwort"
+          placeholderTextColor={currentTheme.textMuted}
+          value={password}
+          onChangeText={setPassword}
           secureTextEntry
-          className="w-full border rounded p-2 mb-4"
+          className="w-full rounded-lg p-4 mb-6"
+          style={{
+            backgroundColor: currentTheme.secondaryBg,
+            color: currentTheme.textPrimary,
+            borderWidth: 1,
+            borderColor: currentTheme.borderPrimary,
+          }}
         />
-        <Pressable className="bg-blue-500 p-3 rounded w-full">
-          <Text className="text-white text-center">Login</Text>
-        </Pressable>
+
+        <AuthButton
+          title="Anmelden"
+          onPress={handleLogin}
+          isLoading={isLoading}
+        />
+
+        <Link href="/register" asChild>
+          <Pressable>
+            <Text style={{ color: currentTheme.chatBot }}>
+              Noch kein Konto? Registrieren
+            </Text>
+          </Pressable>
+        </Link>
       </View>
     </BaseBackground>
   );