Fix Ctrl+Click selection, add Delete key with confirmation dialog

- Fix Ctrl+Click: mousedown no longer immediately adds item, preventing
  onClick from toggling it back off. Drag-select still works via mouseenter.
- Delete key removes selected items/packages with JDownloader-style
  confirmation dialog showing count and remaining items.
- "Nicht mehr anzeigen" checkbox disables future confirmations.
- New setting "Vor dem Löschen bestätigen" under Allgemein.

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

src/main/constants.ts

@@ -75,6 +75,7 @@ export function defaultSettings(): AppSettings {
     theme: "dark" as const,
     collapseNewPackages: true,
     autoSkipExtracted: false,
+    confirmDeleteSelection: true,
     bandwidthSchedules: []
   };
 }