fix(webui): show credential schema load errors

[Premsenareddy]

Summary

• show inline errors when credential provider schema loading fails in create/edit credential dialogs
• add a credential page provider-load error state instead of silently showing no providers
• keep the sidebar in a loading state while provider schemas are still loading

Fixes #1840

Tests

• corepack pnpm exec prettier --check frontend/src/components/dialog/CreateCredentialDialog.tsx frontend/src/components/dialog/EditCredentialDialog.tsx frontend/src/pages/credential/index.tsx frontend/src/i18n/locales/en.json frontend/src/i18n/locales/zh.json
• corepack pnpm --filter frontend build
• corepack pnpm --filter frontend lint

[YingchaoX]

Thanks for working on this.

I tested the credential page locally and found a related failure mode that this PR makes more visible but does not fully prevent: if the setup page stores the server URL as localhost:8000 instead of http://localhost:8000, the API client throws before the request is sent:

TypeError: Failed to construct 'URL': Invalid URL

This still results from the same credential schema loading path, but the root cause is that the setup flow accepts or preserves a browser-unusable server URL.

Would you be open to including a small normalization step in this PR, for example converting localhost:8000 / 127.0.0.1:8000 to http://localhost:8000 before saving server_url?

[Premsenareddy]

Thanks for testing that path. I added the normalization in the follow-up commit:

• localhost:8000 / 127.0.0.1:8000 are normalized to http://... before saving from setup.
• the setup input now allows schemeless local URLs to reach submit instead of relying on native type=url validation.
• API base URL reads also normalize existing stored values, so a previously saved schemeless URL can recover without throwing before the request is sent.

Local checks passed: prettier check, frontend build, and frontend lint.