Add persistent all-time download counter, session vs total stats

- Track totalDownloadedAllTime in settings (persists across restarts)
- Track sessionDownloadedBytes for current app session
- Status bar shows both: Session + Gesamt
- Statistics section shows Heruntergeladen (Session) + (Gesamt)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/shared/types.ts

@@ -28,6 +28,7 @@ export interface BandwidthScheduleEntry {
 
 export interface DownloadStats {
   totalDownloaded: number;
+  totalDownloadedAllTime: number;
   totalFiles: number;
   totalPackages: number;
   sessionStartedAt: number;
@@ -76,6 +77,7 @@ export interface AppSettings {
   collapseNewPackages: boolean;
   autoSkipExtracted: boolean;
   confirmDeleteSelection: boolean;
+  totalDownloadedAllTime: number;
   bandwidthSchedules: BandwidthScheduleEntry[];
 }