feat(openclaw): add default model button and fix sessionsApi export

- Add "Enable as Default" button to OpenClaw provider cards
- Place enable button before add/remove button for better UX
- Disable remove button when provider is set as default
- Add sessionsApi re-export from api barrel file to fix white screen
- Add i18n keys for default model feature (zh/en/ja)

src/lib/api/index.ts

@@ -8,6 +8,7 @@ export { usageApi } from "./usage";
 export { vscodeApi } from "./vscode";
 export { proxyApi } from "./proxy";
 export { openclawApi } from "./openclaw";
+export { sessionsApi } from "./sessions";
 export * as configApi from "./config";
 export type { ProviderSwitchEvent } from "./providers";
 export type { Prompt } from "./prompts";