chore(cleanup): 移除用户中心孤儿签到日历渲染器

credits-signin.php 已无任何引用（用户中心视图映射无 signin 项，profile-credits.php 亦不再挂载），整包删除，不保留兼容分支。

其专属 .uc-signin-calendar__title 样式与 public.css / public.min.css 产物同步移除；签到日历仍由 src/Credits/Signin.php 在签到弹窗内输出，enhancements2.js 的日历渲染逻辑保持不变。

tests/widget-system/toast-contract.test.cjs

@@ -204,7 +204,6 @@ test('balance center actions use direct fixed-id notyf flows', () => {
   const header = read('inc/helpers/user-center-renderers/profile-header.php');
   const headerUserbox = read('inc/helpers/aksrt-header-userbox.php');
   const signinService = read('src/Credits/Signin.php');
-  const signin = read('inc/helpers/user-center-renderers/credits-signin.php');
   const context = read('inc/helpers/aksrt-user-center.php');
   const renderers = [
     'inc/helpers/user-center-renderers/credits-balance.php',
@@ -256,7 +255,6 @@ test('balance center actions use direct fixed-id notyf flows', () => {
   assert.match(headerUserbox, /data-signin-captcha[\s\S]*?aksrt_render_captcha\('signin'\)/);
   assert.match(signinService, /aksrt-checkin-modal__btn[^>]+data-signin-btn/);
   assert.doesNotMatch(headerUserbox + signinService, /data-aksrt-checkin/);
-  assert.doesNotMatch(signin, /data-signin-captcha|aksrt_render_captcha/);
   assert.match(context, /'signin_enabled'\s*=>\s*\$signin_enabled/);
   assert.match(context, /'signin_signed_today'\s*=>\s*\$signin_signed_today/);
   assert.match(shell, /\.uc-header__actions\s*\{[^}]*display:\s*flex;[^}]*gap:\s*6px;/s);