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/Utilities/Extensions/Color+Extensions.swift

@@ -60,10 +60,11 @@ extension Color {
     
     static func providerColor(_ provider: Settings.Provider) -> Color {
         switch provider {
-        case .openrouter: return Color(hex: "#7c3aed")  // Purple
-        case .anthropic: return Color(hex: "#d4895a")    // Orange
-        case .openai: return Color(hex: "#10a37f")       // Green
-        case .ollama: return Color(hex: "#ffffff")       // White
+        case .openrouter:    return Color(hex: "#7c3aed")  // Purple
+        case .anthropic:     return Color(hex: "#d4895a")  // Orange
+        case .openai:        return Color(hex: "#10a37f")  // Green
+        case .ollama:        return Color(hex: "#ffffff")  // White
+        case .appleOnDevice: return Color(hex: "#636366")  // Apple grey
         }
     }