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

Apple Intelligence / Foundation Models is now genuinely available on
this machine (macOS 27 beta 4) — it was reverted back in June
(f63226b) when it wasn't. Ports the reverted AppleFoundationProvider
forward onto 2.4.3, adapted for everything that's changed since:
PolyForm license headers, current AIProvider protocol shape, current
Settings.Provider/ProviderRegistry/CreditsView/SettingsView structure.

Fixes a real bug found via live testing: LanguageModelSession.
GenerationError was deprecated in macOS 27.0 in favor of a new
LanguageModelError type. On a macOS 27+ runtime, generation failures
now throw LanguageModelError, not GenerationError, so the original
error-mapping catch never matched and Apple's raw error text leaked
to the user instead of oAI's friendly message. Now dispatches to
whichever type the runtime actually throws, gated with
@available(macOS 27.0, *), keeping the old GenerationError path as
a fallback for macOS 26.x (the app's actual deployment target).

Confirmed end-to-end in the live app: provider selectable, Settings
shows a live "Available" badge, chat header shows correct branding,
and — a real, expected Phase 1 limitation — oAI's default system
prompt (active Agent Skills + MCP tool guidance, ~16K tokens on this
machine) exceeds the on-device model's 4K context window on the
very first message. The friendly error message now correctly reports
this instead of Apple's raw string. Tool calling remains out of scope
until Phase 3.

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

oAI/Providers/ProviderRegistry.swift

@@ -67,6 +67,9 @@ class ProviderRegistry {
 
         case .ollama:
             provider = OllamaProvider(baseURL: settings.ollamaEffectiveURL)
+
+        case .appleOnDevice:
+            provider = AppleFoundationProvider()
         }
         
         // Cache and return
@@ -104,6 +107,8 @@ class ProviderRegistry {
             return settings.openaiAPIKey != nil && !settings.openaiAPIKey!.isEmpty
         case .ollama:
             return settings.ollamaConfigured
+        case .appleOnDevice:
+            return true  // no API key needed
         }
     }