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/components/AuthButton.tsx

@@ -0,0 +1,41 @@
+import { Pressable, Text, ActivityIndicator } from "react-native";
+import currentTheme from "../Themes";
+
+interface AuthButtonProps {
+  title: string;
+  onPress: () => void;
+  isLoading?: boolean;
+}
+
+const AuthButton = ({ title, onPress, isLoading = false }: AuthButtonProps) => {
+  return (
+    <Pressable
+      onPress={onPress}
+      disabled={isLoading}
+      className="w-full rounded-lg p-4 mb-4 border-4"
+      style={{
+        backgroundColor: isLoading
+          ? currentTheme.disabledButton
+          : currentTheme.chatBot,
+        shadowColor: "#000",
+        shadowOffset: { width: 0, height: 2 },
+        shadowOpacity: 0.25,
+        shadowRadius: 3.84,
+        elevation: 5,
+      }}
+    >
+      {isLoading ? (
+        <ActivityIndicator color={currentTheme.buttonText} />
+      ) : (
+        <Text
+          className="text-center font-semibold text-lg"
+          style={{ color: currentTheme.buttonText }}
+        >
+          {title}
+        </Text>
+      )}
+    </Pressable>
+  );
+};
+
+export default AuthButton;

apps/client/src/components/Header.tsx

@@ -1,12 +1,20 @@
-import { View } from "react-native";
+import { View, Pressable } from "react-native";
+import { Ionicons } from "@expo/vector-icons";
+import { router } from "expo-router";
 import currentTheme from "../Themes";
 import { ReactNode } from "react";
+import { AuthService } from "../services";
 
 type HeaderProps = {
   children?: ReactNode;
   className?: string;
 };
 
+const handleLogout = async () => {
+  await AuthService.logout();
+  router.replace("/login");
+};
+
 const Header = (props: HeaderProps) => {
   return (
     <View>
@@ -17,6 +25,13 @@ const Header = (props: HeaderProps) => {
         }}
       >
         {props.children}
+        <Pressable
+          onPress={handleLogout}
+          className="absolute right-1 top-0 p-2"
+          hitSlop={8}
+        >
+          <Ionicons name="log-out-outline" size={24} color={currentTheme.primeFg} />
+        </Pressable>
       </View>
       <View
         className="h-2 bg-black"