feat(group-chat): add ephemeral visual pass indicator when bot passes turn

- Display a subtle centered system pill '[⏩ {bot} passed]' when a bot passes
- Style with Material icon (Icons.Filled.FastForward) and surface tokens
- Keep pass indicators strictly ephemeral to avoid polluting LLM prompt context
- Exclude from cross-device snapshot sync so it disappears on restart/rehydrate
- Add English and Arabic localized string resources
- Update GroupChatViewModelTest to verify pass indicator presence and LLM filtering

Co-authored-by: Luna <luna.m57@agentmail.to>

app/src/main/res/values-ar/strings.xml

@@ -1168,6 +1168,7 @@
 <string name="group_chat_empty_title">لا توجد رسائل بعد</string>
 <string name="group_chat_empty_subtitle">أشر إلى بوت أو استخدم @all لبدء التعاون.</string>
 <string name="group_chat_thinking">%1$s يفكّر الآن…</string>
+<string name="group_chat_bot_passed">تخطى %1$s دوره</string>
 <string name="group_chat_typing">جارٍ الكتابة…</string>
 <string name="group_chat_tool_running">جارٍ التشغيل…</string>
 <string name="group_chat_tool_no_output">لا يوجد مخرجات</string>