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/tsconfig.json

@@ -0,0 +1,6 @@
+{
+  "extends": "expo/tsconfig.base",
+  "compilerOptions": {
+    "strict": true
+  }
+}