Support SSO login via OIDC

[Jraaay]

This pull request adds support for OIDC (OpenID Connect) authentication to the application, allowing users to log in using an external OIDC provider. The changes include backend support for OIDC settings and authentication flow, frontend integration for OIDC login, and updates to settings management.

A config example are as follow:

[oidc]
ClientId     = clientid123456
ClientSecret = clientsecret123456
Endpoint     = https://example.com/oidc
RedirectUri  = http://localhost:9000/
Scopes       = openid profile email username
Identifier   = username

---

Note

Introduce OIDC-based SSO login, integrating new backend endpoints and settings, frontend login flow, example config, env docs, tests, and dependencies.

• Auth/Backend:
  • Add OIDC flow with GET /oidc_uri and POST /oidc_callback using coreos/go-oidc and golang.org/x/oauth2; validates state via cookie, exchanges code, verifies ID token, maps claims for username, and issues JWT.
  • Introduce settings.OIDC (client_id, client_secret, endpoint, redirect_uri, scopes, identifier); expose in GET/POST /settings, env parsing, and example config; include tests in settings_test.go.
  • Update api/settings/settings.go to read/write oidc and persist; router wires new OIDC endpoints.
  • Dependencies: add github.com/coreos/go-oidc/v3 and direct golang.org/x/oauth2.
• Frontend:
  • API: get_oidc_uri, oidc_login in app/src/api/auth.ts.
  • Login view: OIDC button and callback handling (detects nginx-ui-oidc_ state), strips query params post-login.
  • Settings store: add oidc fields and types.
• Configuration & Docs:
  • Add [oidc] block to app.example.ini.
  • Document OIDC env vars in docs/guide/env.md, docs/zh_CN/guide/env.md, docs/zh_TW/guide/env.md.

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[0xJacky]

@cursor review

[0xJacky]

LGTM 🎉