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/java/com/m57/hermescontrol/assistant/AssistantService.kt

@@ -0,0 +1,10 @@
+package com.m57.hermescontrol.assistant
+
+import android.service.voice.VoiceInteractionService
+
+/**
+ * Top-level digital-assistant service. The system keeps this running while
+ * Hermes Mobile is the default assistant; the actual trigger handling lives
+ * in [AssistantSessionService] / [AssistantSession].
+ */
+class AssistantService : VoiceInteractionService()