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

setting/system_setting/oidc.go

@@ -1,9 +1,14 @@
 package system_setting
 
-import "github.com/QuantumNous/new-api/setting/config"
+import (
+	"strings"
+
+	"github.com/QuantumNous/new-api/setting/config"
+)
 
 type OIDCSettings struct {
 	Enabled               bool   `json:"enabled"`
+	DisplayName           string `json:"display_name"`
 	ClientId              string `json:"client_id"`
 	ClientSecret          string `json:"client_secret"`
 	WellKnown             string `json:"well_known"`
@@ -23,3 +28,14 @@ func init() {
 func GetOIDCSettings() *OIDCSettings {
 	return &defaultOIDCSettings
 }
+
+// GetEffectiveDisplayName returns the admin-configured display name, or the
+// literal "OIDC" when none has been set. Centralizing this fallback keeps the
+// default in one place for both the OAuth provider name and the public
+// status payload.
+func (s *OIDCSettings) GetEffectiveDisplayName() string {
+	if trimmed := strings.TrimSpace(s.DisplayName); trimmed != "" {
+		return trimmed
+	}
+	return "OIDC"
+}