Restore auth tab localization in settings

The settings page already routes the auth tab label through the shared i18n key, but the locale bundles never defined that key. Adding the missing entries fixes the label with the same simple pattern used by the other tabs.

Constraint: Keep the fix aligned with the existing settings-tab i18n flow
Rejected: Add component-level bilingual rendering | unnecessary for a missing translation key
Confidence: high
Scope-risk: narrow
Reversibility: clean
Directive: When adding settings tabs, define locale keys in every bundled language before relying on fallback text
Tested: pnpm format:check; pnpm typecheck
Not-tested: Manual verification in the desktop UI

src/i18n/locales/en.json

@@ -243,6 +243,7 @@
     "title": "Settings",
     "general": "General",
     "tabGeneral": "General",
+    "tabAuth": "Auth",
     "tabAdvanced": "Advanced",
     "tabProxy": "Proxy",
     "authCenter": {

src/i18n/locales/ja.json

@@ -243,6 +243,7 @@
     "title": "設定",
     "general": "一般",
     "tabGeneral": "一般",
+    "tabAuth": "認証",
     "tabAdvanced": "詳細",
     "tabProxy": "プロキシ",
     "authCenter": {

src/i18n/locales/zh.json

@@ -243,6 +243,7 @@
     "title": "设置",
     "general": "通用",
     "tabGeneral": "通用",
+    "tabAuth": "认证",
     "tabAdvanced": "高级",
     "tabProxy": "代理",
     "authCenter": {