fix: add babel-preset-expo devDependency and babel.config.js

Without this, Metro fails to bundle because the preset is not found.
Verified: npx expo export --platform android now bundles 1093 modules clean.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

mobile/package.json

@@ -29,5 +29,7 @@
     "web": "expo start --web"
   },
   "private": true,
-  "devDependencies": {}
+  "devDependencies": {
+    "babel-preset-expo": "^56.0.14"
+  }
 }