fix(mobile): bump Android versionCode to 6 for the 0.0.25 release

Prepare mobile 0.0.25 (#7751) bumped expo.version but left
expo.android.versionCode at 5 — the same value v0.0.24 shipped. The
Android release workflow uses the committed versionCode as-is (it does
not auto-bump), so a 0.0.25 APK would collide with v0.0.24 and Android
would refuse the update for existing users.

Co-authored-by: Orca <help@stably.ai>

mobile/app.json

@@ -75,7 +75,7 @@
       "allowBackup": false,
       "permissions": ["RECORD_AUDIO", "MODIFY_AUDIO_SETTINGS"],
       "package": "com.stably.orca.mobile",
-      "versionCode": 5
+      "versionCode": 6
     },
     "plugins": [
       "expo-router",