Isolate providers per client: add provider targets and a Codex-side form

src/pages/Providers.test.tsx

@@ -41,6 +41,7 @@ const provider = {
   api_key: "sk-x",
   models: { default: "deepseek-chat", sonnet: "deepseek-chat", haiku: "deepseek-chat", opus: "deepseek-chat" },
   enabled_for_claude: false,
+  target: "claude",
   created_at: "2026-10-18T00:00:00Z",
 };
 
@@ -87,6 +88,7 @@ describe("Providers page", () => {
       name: "Kimi",
       base_url: "https://api.moonshot.cn/v1",
       api_key: "sk-kimi",
+      target: "claude",
     });
   });