feat: register as Android default digital assistant (#757) (#758)

* feat: register as Android default digital assistant

* fix: assistant role request fallback + always-attempt click

* remove: assistant role request row from Settings

The system-level assistant registration stays (VoiceInteractionService
+ ROLE_ASSISTANT, set via system Settings -> Default apps), but the
in-app Settings row is dropped: RequestRoleActivity fails to resolve
the caller on a plain startActivity ('Package name cannot be null or
empty'), so the button was a dead end. Removes the row, the
AssistantRole helper, and the now-unused strings.

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Co-authored-by: M57 <hy4ri@users.noreply.github.com>

app/src/main/res/xml/voice_interaction_service.xml

@@ -0,0 +1,14 @@
+<?xml version="1.0" encoding="utf-8"?>
+<!--
+    Registers Hermes Mobile as a candidate for the Android digital-assistant
+    role (Settings > Apps > Default apps > Digital assistant app).
+
+    Both sessionService and recognitionService are REQUIRED by the framework
+    (VoiceInteractionServiceInfo rejects the service with a parse error if
+    either is missing). Recognition is a stub for now — no speech in v1.
+-->
+<voice-interaction-service
+    xmlns:android="http://schemas.android.com/apk/res/android"
+    android:sessionService="com.m57.hermescontrol.assistant.AssistantSessionService"
+    android:recognitionService="com.m57.hermescontrol.assistant.AssistantRecognitionService"
+    android:supportsAssist="true" />