Add the standard Expo metro.config.js

The project was missing it, so Metro fell back to its bare defaults instead of
extending expo/metro-config; this also resolves the failing expo-doctor check.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: aapeliv

app/mobile/metro.config.js

@@ -0,0 +1,6 @@
+// Learn more https://docs.expo.dev/guides/customizing-metro
+const { getDefaultConfig } = require("expo/metro-config");
+
+const config = getDefaultConfig(__dirname);
+
+module.exports = config;