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/ViewModels/ChatViewModel.swift

@@ -424,6 +424,8 @@ Don't narrate future actions ("Let me...") - just use the tools.
     func inferProviderPublic(from modelId: String) -> Settings.Provider? { inferProvider(from: modelId) }
 
     private func inferProvider(from modelId: String) -> Settings.Provider? {
+        // Apple Foundation Models
+        if modelId.hasPrefix("apple-") { return .appleOnDevice }
         // OpenRouter models always contain a "/" (e.g. "anthropic/claude-3-5-sonnet")
         if modelId.contains("/") { return .openrouter }
         // Anthropic direct (e.g. "claude-sonnet-4-5-20250929")