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/Models/Settings.swift

@@ -58,17 +58,25 @@ struct Settings: Codable {
         case anthropic
         case openai
         case ollama
-        
+        case appleOnDevice = "apple_on_device"
+
         var displayName: String {
-            rawValue.capitalized
+            switch self {
+            case .openrouter:     return "OpenRouter"
+            case .anthropic:      return "Anthropic"
+            case .openai:         return "OpenAI"
+            case .ollama:         return "Ollama"
+            case .appleOnDevice:  return "Apple Intelligence"
+            }
         }
-        
+
         var iconName: String {
             switch self {
-            case .openrouter: return "network"
-            case .anthropic: return "brain"
-            case .openai: return "sparkles"
-            case .ollama: return "server.rack"
+            case .openrouter:     return "network"
+            case .anthropic:      return "brain"
+            case .openai:         return "sparkles"
+            case .ollama:         return "server.rack"
+            case .appleOnDevice:  return "apple.logo"
             }
         }
     }