Feature: Thumbnail Cache Improvements & Critical Fixes

Comprehensive update addressing thumbnail caching, avatar display, and login state management:

## Thumbnail & Avatar Caching
- Dynamic credential retrieval for thumbnail loading
- Account-specific ImageLoaders with shared caches
- Removed deprecated AvatarManager

## Avatar Display Fixes
- Fixed invisible avatars
- Moved avatar loading off main thread
- Fixed startup crashes

## Build Improvements
- Removed duplicate methods
- Code cleanup and refactoring

Author: zerox80

opencloudApp/src/main/java/eu/opencloud/android/MainApp.kt

@@ -39,7 +39,7 @@ import android.widget.CheckBox
 import androidx.appcompat.app.AlertDialog
 import androidx.core.content.pm.PackageInfoCompat
 import eu.opencloud.android.data.providers.implementation.OCSharedPreferencesProvider
-import eu.opencloud.android.datamodel.ThumbnailsCacheManager
+
 import eu.opencloud.android.db.PreferenceManager
 import eu.opencloud.android.dependecyinjection.commonModule
 import eu.opencloud.android.dependecyinjection.localDataSourceModule
@@ -107,8 +107,7 @@ class MainApp : Application() {
 
         SingleSessionManager.setUserAgent(userAgent)
 
-        // initialise thumbnails cache on background thread
-        ThumbnailsCacheManager.InitDiskCacheTask().execute()
+
 
         initDependencyInjection()