Mark Apple Intelligence provider as Beta throughout the UI

Apple's Foundation Models framework is still under active development
(built against macOS 27 beta) and likely to stay rough for a while —
small 4K context window, occasional generation errors, no tool
support yet. Surface that clearly wherever it appears:
- Model name: "Apple On-Device (Beta)" (shows in header/model picker)
- Model description: notes the beta status and what to expect
- Settings -> General: "⚠️ Beta — ..." disclaimer under the Apple
  Intelligence section, same style as the existing Paperless-NGX beta
  note
- Credits panel: same disclaimer for the Apple Intelligence entry

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

oAI/Views/Screens/CreditsView.swift

@@ -85,6 +85,9 @@ struct CreditsView: View {
                         Text("On-device and free — no credits or API key needed.")
                             .font(.body)
                             .foregroundColor(.secondary)
+                        Text("⚠️ Beta — Apple's Foundation Models framework is still in active development.")
+                            .font(.caption)
+                            .foregroundColor(.secondary)
                         Image(systemName: "apple.logo")
                             .font(.system(size: 40))
                             .foregroundColor(.secondary)