fix(mobile): scope Expo exports to native platforms

A bare expo export (used by the OTA preflight and by eoas publish) also
exported the unused web target, whose static render breaks under SDK 56.
XPREM serves only iOS and Android bundles and the website is apps/web, so
declare the Expo app native-only.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

apps/mobile/app.json

@@ -7,6 +7,10 @@
     "icon": "./assets/icon.png",
     "scheme": "app.sauci",
     "userInterfaceStyle": "automatic",
+    "platforms": [
+      "ios",
+      "android"
+    ],
     "ios": {
       "supportsTablet": false,
       "bundleIdentifier": "com.sauci.app",