feat(oidc): 支持自定义 OIDC 登录显示名称 (#6012)

* docs: add design spec for OIDC custom display name

Mirrors the existing Custom OAuth Provider name pattern so admins can
show a meaningful label instead of the hardcoded "OIDC" on the login
page and in related copy.

* feat(oidc): add configurable display name with OIDC fallback

* feat(oidc): use configured display name in provider name and status API

* feat(oidc): add display name field to default-theme OIDC settings

Claude-Session: https://claude.ai/code/session_01FDkWJqigJi9yE3HG5pjZP5

* feat(oidc): show configured display name on default-theme login button

* feat(oidc): add display name field to classic-theme OIDC settings

* feat(oidc): show configured display name on classic-theme login button

* fix(oidc): trim whitespace before applying display name fallback

* chore: remove internal design doc from PR

Design/planning docs are working artifacts for this session and
shouldn't be submitted to the upstream project.

* fix(oidc): lead with example in classic-theme display name placeholder

Reorders the combined placeholder to show the example first, then
the fallback note, matching the Custom OAuth Provider Name field's
placeholder convention (example-only) that this feature mirrors.

* fix(i18n): improve Russian grammar in OIDC display-name placeholder translation

Leads each clause with its condition/subject and adds the missing
verb, per PR review feedback.

* test(web): remove redundant OIDC harness tests

oauth/oidc.go

@@ -41,7 +41,7 @@ type oidcUser struct {
 }
 
 func (p *OIDCProvider) GetName() string {
-	return "OIDC"
+	return system_setting.GetOIDCSettings().GetEffectiveDisplayName()
 }
 
 func (p *OIDCProvider) IsEnabled() bool {