Encrypted backup system + hide extracted items in package list

Backup redesign (JDownloader 2 style):
- AES-256-GCM encrypted .mdd format with fixed app key
- All credentials exported (no more ***-masking), works on any PC
- Includes settings, session AND history in backup
- Backward-compatible: auto-detects legacy JSON backups
- Normalize history entries on import
- Added debridLinkApiKeys, linkSnappy credentials to sensitive keys

Hide extracted items:
- New setting: hide completed/extracted items from package item list
- Items still count in progress stats (done/total), only hidden in UI
- Default: enabled
- Toggle in settings: "Entpackte Items in Paketliste ausblenden"

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

src/main/constants.ts

@@ -99,6 +99,7 @@ export function defaultSettings(): AppSettings {
     accountListShowDetailedDebridLinkKeys: false,
     autoSortPackagesByProgress: true,
     autoSkipExtracted: false,
+    hideExtractedItems: true,
     confirmDeleteSelection: true,
     totalDownloadedAllTime: 0,
     bandwidthSchedules: [],