chore(mobile): add eas.json build profile config

Minimal EAS Build config (development/preview/production profiles) —
needed by `eas credentials` to manage the FCM push key, no EAS Build
runs yet. Push notifications are now fully verified end-to-end: real
device token minted, registered with the backend, and a live push
delivered to the Android emulator's notification tray.

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

mobile/eas.json

@@ -0,0 +1,19 @@
+{
+  "cli": {
+    "version": ">= 23.2.0",
+    "appVersionSource": "local"
+  },
+  "build": {
+    "development": {
+      "developmentClient": true,
+      "distribution": "internal"
+    },
+    "preview": {
+      "distribution": "internal"
+    },
+    "production": {}
+  },
+  "submit": {
+    "production": {}
+  }
+}