fix(types): add 'checking' status to ProviderItem type

The AcpProviderInfo type has a 'checking' status for when provider
availability is being checked, but ProviderItem was missing this
state, causing a type error in chat-panel.tsx.

This fixes the build failure:
  Type 'AcpProviderInfo[]' is not assignable to type 'ProviderItem[]'

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

Author: phodal

src/client/components/tiptap-input.tsx

@@ -437,7 +437,7 @@ interface ProviderItem {
   name: string;
   description: string;
   command: string;
-  status?: "available" | "unavailable";
+  status?: "available" | "unavailable" | "checking";
   /** Source of the provider: "static" for builtin, "registry" for ACP registry */
   source?: "static" | "registry";
 }