Add configurable slicer preference — Bambu Studio or OrcaSlicer (#313)

Users can now choose their preferred slicer (Bambu Studio or OrcaSlicer)
in Settings → General. All "Open in Slicer" buttons across Archives,
3D Preview, and context menus use the correct protocol for the selected
slicer. OrcaSlicer uses orcaslicer://open?file=<URL> on all platforms;
Bambu Studio uses bambustudio:// (Windows) or bambustudioopen:// (macOS/
Linux). Default remains Bambu Studio for backward compatibility.

frontend/src/api/client.ts

@@ -772,6 +772,8 @@ export interface AppSettings {
   library_disk_warning_gb: number;
   // Camera view settings
   camera_view_mode: 'window' | 'embedded';
+  // Preferred slicer
+  preferred_slicer: 'bambu_studio' | 'orcaslicer';
   // Prometheus metrics
   prometheus_enabled: boolean;
   prometheus_token: string;