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.json

@@ -0,0 +1,30 @@
+{
+  "expo": {
+    "name": "mobile",
+    "slug": "mobile",
+    "version": "1.0.0",
+    "orientation": "portrait",
+    "icon": "./assets/icon.png",
+    "userInterfaceStyle": "light",
+    "newArchEnabled": true,
+    "splash": {
+      "image": "./assets/splash-icon.png",
+      "resizeMode": "contain",
+      "backgroundColor": "#ffffff"
+    },
+    "ios": {
+      "supportsTablet": true
+    },
+    "android": {
+      "adaptiveIcon": {
+        "foregroundImage": "./assets/adaptive-icon.png",
+        "backgroundColor": "#ffffff"
+      },
+      "edgeToEdgeEnabled": true,
+      "predictiveBackGestureEnabled": false
+    },
+    "web": {
+      "favicon": "./assets/favicon.png"
+    }
+  }
+}