feat: Firebase Cloud Messaging push notifications

Backend:
- New user_fcm_tokens table (migration 010) for storing device tokens
- FCMService sends push via Firebase HTTP legacy API (server key from .env)
- POST /api/fcm/register endpoint for Android to register its FCM token
- AdminController::sendNotification() triggers FCM push after DB insert
- FCM config added to config.php

Android:
- Firebase Cloud Messaging service (ServerManagerFirebaseService)
  receives push notifications and shows them via NotificationHelper
- FCM token registered with backend on new token and on login
- google-services.json template (must be replaced with real Firebase config)
- firebase-setup.sh script with configuration instructions
- FCM dependency + google-services plugin added to Gradle
- Bump to v1.8.0 (versionCode 12)

android/build.gradle.kts

@@ -2,4 +2,5 @@ plugins {
     id("com.android.application") version "9.2.1" apply false
     id("org.jetbrains.kotlin.plugin.compose") version "2.3.21" apply false
     id("org.jetbrains.kotlin.plugin.serialization") version "2.3.21" apply false
+    id("com.google.gms.google-services") version "4.4.2" apply false
 }