i18n: offer Arabic in the language picker

The Weblate merge added a values-ar translation (~50% complete, RTL —
the app already declares supportsRtl). Add the matching <locale> line so
Arabic is selectable in both the in-app picker and the system per-app
language settings. Russian and Portuguese stay out for now: at 12% and
<1% they're below the usable bar (the lowest currently-offered language,
zh-CN, sits at ~24%).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

app/src/main/res/xml/locales_config.xml

@@ -9,6 +9,7 @@
 -->
 <locale-config xmlns:android="http://schemas.android.com/apk/res/android">
     <locale android:name="en" />
+    <locale android:name="ar" />
     <locale android:name="de" />
     <locale android:name="es" />
     <locale android:name="fr" />