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/main/constants.ts

@@ -76,6 +76,7 @@ export function defaultSettings(): AppSettings {
     collapseNewPackages: true,
     autoSkipExtracted: false,
     confirmDeleteSelection: true,
+    totalDownloadedAllTime: 0,
     bandwidthSchedules: []
   };
 }