feat: Expo scaffold + auth screens + core screens (#39-#46, #48)

components/AppButton.tsx

@@ -0,0 +1,49 @@
+import React from 'react';
+import { TouchableOpacity, Text, ActivityIndicator, View } from 'react-native';
+
+interface AppButtonProps {
+  title: string;
+  onPress: () => void;
+  variant?: 'primary' | 'secondary' | 'danger' | 'outline';
+  loading?: boolean;
+  disabled?: boolean;
+  className?: string;
+}
+
+export const AppButton: React.FC<AppButtonProps> = ({
+  title,
+  onPress,
+  variant = 'primary',
+  loading = false,
+  disabled = false,
+  className = '',
+}) => {
+  const baseClasses = 'flex-row items-center justify-center rounded-xl px-6 py-3.5 active:opacity-80';
+  const variantClasses = {
+    primary: 'bg-blue-600',
+    secondary: 'bg-gray-200',
+    danger: 'bg-red-600',
+    outline: 'border-2 border-blue-600 bg-transparent',
+  };
+  const textClasses = {
+    primary: 'text-white font-semibold text-base',
+    secondary: 'text-gray-800 font-semibold text-base',
+    danger: 'text-white font-semibold text-base',
+    outline: 'text-blue-600 font-semibold text-base',
+  };
+  const disabledClass = disabled || loading ? 'opacity-50' : '';
+
+  return (
+    <TouchableOpacity
+      onPress={onPress}
+      disabled={disabled || loading}
+      className={`${baseClasses} ${variantClasses[variant]} ${disabledClass} ${className}`}
+    >
+      {loading ? (
+        <ActivityIndicator color={variant === 'outline' ? '#2563EB' : '#fff'} size="small" />
+      ) : (
+        <Text className={textClasses[variant]}>{title}</Text>
+      )}
+    </TouchableOpacity>
+  );
+};