Add Apple Intelligence provider (Phase 1 — on-device)

- New AppleFoundationProvider using FoundationModels framework (macOS 27+)
- Streaming via streamResponse(to:) → ResponseStream<String> snapshot deltas
- Session built with system prompt + conversation history injected as instructions text
- Full error mapping: context exceeded, guardrail violation, rate limit, availability states
- Settings.Provider.appleOnDevice case wired through ProviderRegistry, Color+Extensions, CreditsView
- inferProvider() detects "apple-" prefix model IDs
- Settings → General: Apple Intelligence section with live availability badge and deep link to System Settings

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

oAI/Views/Screens/CreditsView.swift

@@ -80,6 +80,17 @@ struct CreditsView: View {
                             .font(.system(size: 40))
                             .foregroundColor(.green)
                             .padding(.top)
+
+                    case .appleOnDevice:
+                        Text("Apple Intelligence")
+                            .font(.headline)
+                        Text("On-device and free — no credits or API key needed.")
+                            .font(.body)
+                            .foregroundColor(.secondary)
+                        Image(systemName: "apple.logo")
+                            .font(.system(size: 40))
+                            .foregroundColor(.secondary)
+                            .padding(.top)
                     }
                 }