Fix registration validation + mobile modal scroll

- Email validation (must contain @, min 5 chars)
- Name validation (min 2 chars)
- Modal: max-height 90dvh, overflow-y auto, sticky buttons
- TaskForm: sticky bottom action buttons for mobile keyboard

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

mobile/App.tsx

@@ -0,0 +1,20 @@
+import { StatusBar } from 'expo-status-bar';
+import { StyleSheet, Text, View } from 'react-native';
+
+export default function App() {
+  return (
+    <View style={styles.container}>
+      <Text>Open up App.tsx to start working on your app!</Text>
+      <StatusBar style="auto" />
+    </View>
+  );
+}
+
+const styles = StyleSheet.create({
+  container: {
+    flex: 1,
+    backgroundColor: '#fff',
+    alignItems: 'center',
+    justifyContent: 'center',
+  },
+});