M12e follow-up: FcmMessageBody uses fid (D-096)

The push examples carry message.fid since c694f78; the :data DTO still
named the field token, so ExampleRoundTripTest failed on main.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TQd3pEP9UeN2uifbHknQ7B

android/data/src/main/kotlin/se/hagstedt/kvitt/data/api/Endpoints.kt

@@ -169,7 +169,8 @@ data class FcmMessageDto(val message: FcmMessageBody)
 
 @Serializable
 data class FcmMessageBody(
-    val token: String,
+    /** The target Firebase Installation ID (HTTP v1 `message.fid`, D-096). */
+    val fid: String,
     val android: FcmAndroidConfig,
     val data: Map<String, String>,
 )