feat: add reusable LoadingState component with TDD

Add minimal LoadingState component for displaying loading states across the app. Includes ActivityIndicator, optional message text, theme support, and comprehensive test coverage.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

components/LoadingState.tsx

@@ -0,0 +1,17 @@
+import React from 'react'
+import { View, ActivityIndicator, ViewProps } from 'react-native'
+import Text from './ui/Text'
+
+interface LoadingStateProps extends ViewProps {
+  message?: string
+  color?: string
+}
+
+export default function LoadingState({ message, color = '#F5F5F5', testID, ...props }: LoadingStateProps) {
+  return (
+    <View testID={testID} className="flex-1 items-center justify-center" {...props}>
+      <ActivityIndicator size="large" color={color} />
+      {message && <Text className="mt-4">{message}</Text>}
+    </View>
+  )
+}