Migrate flutter_markdown → flutter_markdown_plus; fix app label

flutter_markdown was discontinued; flutter_markdown_plus is the
maintained drop-in replacement (same API, updated imports only).

Also set android:label to "Fabled" so the app appears correctly
on the home screen and in the app drawer instead of "fabled_app".

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

android/app/src/main/AndroidManifest.xml

@@ -2,7 +2,7 @@
     <uses-permission android:name="android.permission.REQUEST_INSTALL_PACKAGES" />
 
     <application
-        android:label="fabled_app"
+        android:label="Fabled"
         android:name="${applicationName}"
         android:icon="@mipmap/ic_launcher"
         android:usesCleartextTraffic="true">