fix: update Sonnet model label from 4.5 to 4.6

- src/app/api/providers/models/route.ts: updated DEFAULT_MODELS and
  OpenRouter provider mapping from "Sonnet 4.5" to "Sonnet 4.6"
- src/components/chat/MessageInput.tsx: updated DEFAULT_MODEL_OPTIONS
  fallback label from "Sonnet 4.5" to "Sonnet 4.6"

src/components/chat/MessageInput.tsx

@@ -117,7 +117,7 @@ const MODE_OPTIONS: ModeOption[] = [
 
 // Default Claude model options — used as fallback when API is unavailable
 const DEFAULT_MODEL_OPTIONS = [
-  { value: 'sonnet', label: 'Sonnet 4.5' },
+  { value: 'sonnet', label: 'Sonnet 4.6' },
   { value: 'opus', label: 'Opus 4.6' },
   { value: 'haiku', label: 'Haiku 4.5' },
 ];