feat(immich): add prefetch and mobile data controls

Adds Immich image prefetch and a mobile data option to the UI.
Android now ships ImmichPrefetchWorker to fetch originals in the background.
Frontend and desktop code are updated to expose and persist new settings.
Translations cover the new labels and hints in multiple languages.

- Introduce ImmichPrefetchWorker for background prefetch
- Add allowMobileData and prefetchImmich UI controls
- Wire changes to desktop state and translations

plugins/android/src/main/java/ImmichPrefetchWorker.kt

@@ -0,0 +1,37 @@
+package de.wechselbild.wallpaper
+
+import android.content.Context
+import androidx.work.Constraints
+import androidx.work.ExistingPeriodicWorkPolicy
+import androidx.work.NetworkType
+import androidx.work.PeriodicWorkRequestBuilder
+import androidx.work.WorkManager
+import androidx.work.Worker
+import androidx.work.WorkerParameters
+import java.util.concurrent.TimeUnit
+
+class ImmichPrefetchWorker(context: Context, parameters: WorkerParameters) : Worker(context, parameters) {
+  override fun doWork(): Result {
+    WallpaperStore.prefetchImmichOriginals(applicationContext)
+    return Result.success()
+  }
+
+  companion object {
+    private const val WORK_NAME = "immich-wifi-prefetch"
+
+    fun enqueue(context: Context) {
+      val constraints = Constraints.Builder()
+        .setRequiredNetworkType(NetworkType.UNMETERED)
+        .setRequiresBatteryNotLow(true)
+        .build()
+      val request = PeriodicWorkRequestBuilder<ImmichPrefetchWorker>(6, TimeUnit.HOURS)
+        .setConstraints(constraints)
+        .build()
+      WorkManager.getInstance(context).enqueueUniquePeriodicWork(WORK_NAME, ExistingPeriodicWorkPolicy.KEEP, request)
+    }
+
+    fun cancel(context: Context) {
+      WorkManager.getInstance(context).cancelUniqueWork(WORK_NAME)
+    }
+  }
+}