PERF: Replace slow `:has` selectors with body classes (#29581)

app/assets/javascripts/discourse/app/templates/login.hbs

@@ -6,7 +6,7 @@
 }}
   {{hide-application-header-buttons "search" "login" "signup" "menu"}}
   {{hide-application-sidebar}}
-
+  {{body-class "login-page"}}
   <div class="login-fullpage">
     <FlashMessage @flash={{this.flash}} @type={{this.flashType}} />
 

app/assets/javascripts/discourse/app/templates/signup.hbs

@@ -1,7 +1,7 @@
 {{! template-lint-disable no-duplicate-id }}
 {{hide-application-header-buttons "search" "login" "signup" "menu"}}
 {{hide-application-sidebar}}
-
+{{body-class "signup-page"}}
 <div class="signup-fullpage">
   <FlashMessage @flash={{this.flash}} @type={{this.flashType}} />
 

app/assets/stylesheets/common/base/login-signup-page.scss

@@ -9,14 +9,17 @@
   flex-direction: column;
 }
 
-#main-outlet:has(.login-fullpage, .signup-fullpage, .invites-show) {
+body.login-page,
+body.signup-page,
+body.invite-page {
   & ~ .powered-by-discourse,
   .above-main-container-outlet {
     display: none;
   }
 }
 
-body:has(.login-fullpage, .signup-fullpage) {
+body.login-page,
+body.signup-page {
   background-color: var(--secondary);
 }