feat: scaffold ShieldAI React Native mobile app MVP (FRE-4572)

Build complete Expo/React Native mobile app with:
- Auth flow: email/password login, registration, biometric auth
- Dashboard: exposure summary, spam stats, voice protection status
- DarkWatch: watch list management, exposure feed, alert toggles
- SpamShield: call/text history, whitelist/blacklist management
- VoicePrint: family member enrollment, voice analysis
- Settings: tier management, notification preferences, security
- Push notification integration via FCM/APNs
- Offline-first state management with Zustand + AsyncStorage
- Integration with @shieldai/mobile-api-client for API services
- React Navigation with auth-aware routing (stack + bottom tabs)
- Dark theme with consistent design system (colors, spacing, typography)
- Network status monitoring and offline request queuing

Co-Authored-By: Paperclip <noreply@paperclip.ing>

packages/mobile/src/components/Button.tsx

@@ -0,0 +1,65 @@
+import React from 'react';
+import { StyleSheet, Text, TouchableOpacity, View } from 'react-native';
+import { COLORS, BORDER_RADIUS, FONT_SIZES } from '@/constants/theme';
+
+interface ButtonProps {
+  title: string;
+  onPress: () => void;
+  variant?: 'primary' | 'secondary' | 'danger' | 'ghost';
+  disabled?: boolean;
+  loading?: boolean;
+  fullWidth?: boolean;
+}
+
+export function Button({
+  title,
+  onPress,
+  variant = 'primary',
+  disabled = false,
+  loading = false,
+  fullWidth = false,
+}: ButtonProps) {
+  const variantColors = {
+    primary: { bg: COLORS.primary, text: '#fff' },
+    secondary: { bg: COLORS.secondary, text: '#fff' },
+    danger: { bg: COLORS.danger, text: '#fff' },
+    ghost: { bg: 'transparent', text: COLORS.primary },
+  };
+
+  const colors = variantColors[variant];
+
+  return (
+    <TouchableOpacity
+      style={[
+        styles.button,
+        { backgroundColor: colors.bg, opacity: disabled || loading ? 0.5 : 1 },
+        fullWidth && styles.fullWidth,
+      ]}
+      onPress={onPress}
+      disabled={disabled || loading}
+      activeOpacity={0.7}
+    >
+      <Text style={[styles.text, { color: colors.text }]}>
+        {loading ? '...' : title}
+      </Text>
+    </TouchableOpacity>
+  );
+}
+
+const styles = StyleSheet.create({
+  button: {
+    borderRadius: BORDER_RADIUS.md,
+    paddingVertical: 12,
+    paddingHorizontal: 24,
+    alignItems: 'center',
+    justifyContent: 'center',
+    marginVertical: 4,
+  },
+  fullWidth: {
+    width: '100%',
+  },
+  text: {
+    fontSize: FONT_SIZES.md,
+    fontWeight: '600',
+  },
+});