fix: IME layout, message dedup, animation order, and overlay input positioning

- ChatScreen: restructure to Box overlay layout so only input area
  rises with IME, messages stay fixed. Add opaque background to input
  area. Use reverseLayout with newest-first animation order.
- OverlayContent: remove all manual IME detection — system forces
  adjust=pan on VoiceInteractionSession windows, so manual padding
  caused double offset. Let system handle IME naturally.
- ChatRepositoryImpl: add messageRemovals flow to clean up wrapping
  stream_end/response when review/multi_message items arrive later.
  Track lastResponseId in both stream_end and response handlers.
- ChatViewModel/OverlayViewModel: fix dedup to check by message ID
  only. Sort descending (newest first). Observe messageRemovals.
- NavGraph: keep all tabs composed with graphicsLayer alpha toggle —
  prevents ChatScreen destruction and re-render on tab switch.
- CyreneVoiceInteractionSession: defer configureWindow via post()
  to override system softInputMode flags.
- AndroidManifest: set windowSoftInputMode=adjustNothing on main
  activity.
- Add WebSocketKeepAliveService for background connection persistence.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

app/src/main/AndroidManifest.xml

@@ -14,6 +14,7 @@
     <uses-permission android:name="android.permission.FOREGROUND_SERVICE" />
     <uses-permission android:name="android.permission.FOREGROUND_SERVICE_MICROPHONE" />
     <uses-permission android:name="android.permission.FOREGROUND_SERVICE_SPECIAL_USE" />
+    <uses-permission android:name="android.permission.FOREGROUND_SERVICE_DATA_SYNC" />
 
     <!-- 推送 -->
     <uses-permission android:name="android.permission.POST_NOTIFICATIONS" />
@@ -42,6 +43,7 @@
             android:name=".MainActivity"
             android:exported="true"
             android:launchMode="singleTask"
+            android:windowSoftInputMode="adjustNothing"
             android:theme="@style/Theme.Cyrene">
             <intent-filter>
                 <action android:name="android.intent.action.MAIN" />
@@ -83,6 +85,12 @@
                 android:resource="@xml/accessibility_config" />
         </service>
 
+        <!-- WebSocket 后台保活 -->
+        <service
+            android:name=".service.WebSocketKeepAliveService"
+            android:exported="false"
+            android:foregroundServiceType="dataSync" />
+
         <!-- FileProvider：日志分享 -->
         <provider
             android:name="androidx.core.content.FileProvider"