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/index.ts

@@ -0,0 +1,8 @@
+import { registerRootComponent } from 'expo';
+
+import App from './App';
+
+// registerRootComponent calls AppRegistry.registerComponent('main', () => App);
+// It also ensures that whether you load the app in Expo Go or in a native build,
+// the environment is set up appropriately
+registerRootComponent(App);