feat(a11y): 设置页无障碍审计并修复开关可访问名称

axe 结构性规则扩展到设置页（安全 Tab + 其他 Tab）：
- security-tab 8 个 Switch/SelectTrigger 补可访问名称（Label htmlFor 关联 + aria-label）
- other-tab / auth-source-modal 各 1 个未命名 Switch 补 aria-label
- 新增 2 个 axe 测试，vitest 126 全绿
- 翻译键 loginCaptchaEnabled 加入 fragments（admin.zh-CN/en）

frontend/messages/fragments/admin.en.json

@@ -320,6 +320,7 @@
       "confirmDelete": "Confirm delete",
       "deleting": "Deleting...",
       "noAuthSources": "No authentication sources configured. Click the button above to add one.",
+      "loginCaptchaEnabled": "Login CAPTCHA",
       "captchaConfig": "CAPTCHA configuration (Cap CAPTCHA)",
       "captchaConfigDesc": "Configure Proof-of-Work (PoW) based invisible CAPTCHA to protect login from brute-force and credential-stuffing attacks",
       "autoStartSolving": "Start solving automatically when page loads",

frontend/messages/fragments/admin.zh-CN.json

@@ -320,6 +320,7 @@
       "confirmDelete": "确认删除",
       "deleting": "删除中...",
       "noAuthSources": "暂无配置的认证源，点击上方按钮新增",
+      "loginCaptchaEnabled": "登录人机验证",
       "captchaConfig": "人机验证配置 (Cap CAPTCHA)",
       "captchaConfigDesc": "配置基于 Proof-of-Work (PoW) 的无感人机验证，保护系统登录免受暴力破解和撞库攻击",
       "autoStartSolving": "打开页面后自动开始计算",