fix: prevent assistant launch from bringing MainActivity to foreground

Add empty taskAffinity to AssistantActivity so it runs in a separate
task from the main app, preventing MainActivity from surfacing when
AssistantActivity finishes.

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

android/app/src/main/AndroidManifest.xml

@@ -56,6 +56,7 @@
             android:name=".assistant.AssistantActivity"
             android:exported="true"
             android:excludeFromRecents="true"
+            android:taskAffinity=""
             android:launchMode="singleTask"
             android:theme="@android:style/Theme.NoDisplay">
             <intent-filter>