feat: 新增皮肤站原生登录与验证码弹窗

登录入口原先会切到主页并打开内嵌网页。改为弹出启动器自己的登录弹窗：

- SkinSiteLoginModal：账号密码校验规则与皮肤站一致，登录前走验证码，
  成功后保存 JWT 并刷新会话。
- SkinSiteCaptchaModal：拉取旋转图片验证码，用滑块调整角度后提交。
- AccountsCard 的登录按钮改为打开该弹窗，登录成功后重新拉取玩家列表。
- 修正 verifyCaptcha 的参数：服务端校验的是旋转角度 degree，不是文字答案。

apps/app-frontend/src/helpers/skin-site-api.ts

@@ -1,5 +1,4 @@
 import { fetch as tauriFetch } from '@tauri-apps/plugin-http'
-
 export const SKIN_SITE_ORIGIN = 'https://skin.starlight.cool'
 
 /** Where the skin site stores the JWT for this launcher. Kept separate from
@@ -204,11 +203,10 @@ export function requestCaptcha(): Promise<SkinSiteResponse<SkinSiteCaptchaRequir
 	return skinSiteRequest<SkinSiteCaptchaRequire>('captcha', { auth: false })
 }
 
-/** Submit the captcha answer; resolves to the clientId to use for login/register. */
+/** Submit the captcha rotation angle; resolves to the clientId used for login/register. */
 export function verifyCaptcha(payload: {
 	clientId: string
-	answer: string
-	[key: string]: unknown
+	degree: number
 }): Promise<SkinSiteResponse<string>> {
 	return skinSiteRequest<string>('verifyCaptcha', { method: 'POST', body: payload, auth: false })
 }