feat: add movie download support using Media3 DownloadService

Implement media downloading with foreground notification showing progress
percentage and speed. Uses Media3's DownloadManager + SimpleCache with
OkHttp datasource for authenticated Jellyfin downloads. Downloaded movies
are saved to the offline database and the player reads from cache
automatically via CacheDataSource.

app/src/main/AndroidManifest.xml

@@ -1,6 +1,10 @@
 <?xml version="1.0" encoding="utf-8"?>
 <manifest xmlns:android="http://schemas.android.com/apk/res/android">
 
+    <uses-permission android:name="android.permission.FOREGROUND_SERVICE" />
+    <uses-permission android:name="android.permission.FOREGROUND_SERVICE_DATA_SYNC" />
+    <uses-permission android:name="android.permission.POST_NOTIFICATIONS" />
+
     <application
         android:name=".PurefinApplication"
         android:allowBackup="true"
@@ -28,6 +32,10 @@
             android:screenOrientation="landscape"
             android:exported="false"
             android:theme="@style/Theme.Purefin" />
+        <service
+            android:name=".download.PurefinDownloadService"
+            android:exported="false"
+            android:foregroundServiceType="dataSync" />
     </application>
 
 </manifest>