fix: normalize provider ID to anthropic (was firstParty causing model validation mismatch)

The subprocess returns apiProvider='firstParty' for direct Anthropic access,
but the adapter hardcodes providerID='anthropic' in SSE events. This mismatch
causes the frontend's isModelValid check to fail, clearing the model selector.

src/server/routes/provider.ts

@@ -20,7 +20,9 @@ function getModels(initData: InitData | null): ModelInfo[] {
 }
 
 function getProviderId(initData: InitData | null): string {
-  return initData?.account?.apiProvider ?? 'anthropic'
+  const p = initData?.account?.apiProvider
+  if (!p || p === 'firstParty') return 'anthropic'
+  return p
 }
 
 function getProviderName(initData: InitData | null): string {