UX: improve back button styles, add optional models (#37588)

Updates our back button styles and adds the ability to pass models. 

Before:
<img width="240" alt="image"
src="https://github.com/user-attachments/assets/c00354d2-b632-4232-9568-adf74f3bd67e"
/>


After:
<img width="240" alt="image"
src="https://github.com/user-attachments/assets/f162cf84-64d5-4f49-b319-124d37c68c00"
/>

Applies globally, so impacts existing uses (mostly admin) 

Before:

<img width="240" alt="image"
src="https://github.com/user-attachments/assets/49d7eaeb-dda4-4380-a01b-d81242a9665a"
/>


After:
<img width="240" alt="image"
src="https://github.com/user-attachments/assets/aed9ab7e-268f-4b21-9cfb-d749a6210b54"
/>

app/assets/stylesheets/common/components/buttons.scss

@@ -470,10 +470,6 @@
       color: var(--primary);
     }
   }
-
-  &.back-button {
-    margin-bottom: 1em;
-  }
 }
 
 @mixin btn-colors($btn-type) {
@@ -538,6 +534,13 @@
   &.--success {
     @include btn-colors("success");
   }
+
+  &.back-button {
+    --d-button-transparent-text-color: var(--tertiary);
+    --d-button-transparent-icon-color: var(--tertiary);
+    margin-bottom: 1em;
+    padding-left: 0;
+  }
 }
 
 .btn-mini-toggle {

frontend/discourse/admin/templates/edit-category/tabs-horizontal.gjs

@@ -1,27 +1,26 @@
 import { concat } from "@ember/helper";
 import { on } from "@ember/modifier";
 import EditCategoryTab from "discourse/admin/components/edit-category-tab";
+import BackButton from "discourse/components/back-button";
 import BreadCrumbs from "discourse/components/bread-crumbs";
-import DBreadcrumbsItem from "discourse/components/d-breadcrumbs-item";
 import DPageHeader from "discourse/components/d-page-header";
 import DToggleSwitch from "discourse/components/d-toggle-switch";
 import { and } from "discourse/truth-helpers";
-import { i18n } from "discourse-i18n";
 
 const EditCategoryTabsHorizontalTemplate = <template>
+  {{#if (and @controller.site.desktopView @controller.model.id)}}
+    <BackButton
+      @route="discovery.category"
+      @model={{concat @controller.parentParams.slug "/" @controller.model.id}}
+      @label="category.back"
+    />
+  {{/if}}
   <DPageHeader
     @titleLabel={{@controller.baseTitle}}
     @showDrawer={{true}}
     @collapseActionsOnMobile={{false}}
   >
-    <:breadcrumbs>
-      {{#if (and @controller.site.desktopView @controller.model.id)}}
-        <DBreadcrumbsItem
-          @path={{concat "/c/" @controller.model.slug}}
-          @label={{i18n "category.back"}}
-        />
-      {{/if}}
-    </:breadcrumbs>
+
     <:actions>
       <DToggleSwitch
         @label="category.show_advanced"

frontend/discourse/app/components/back-button.gjs

@@ -1,10 +1,15 @@
+import { array } from "@ember/helper";
 import { LinkTo } from "@ember/routing";
 import icon from "discourse/helpers/d-icon";
 import { or } from "discourse/truth-helpers";
 import { i18n } from "discourse-i18n";
 
 <template>
-  <LinkTo class="btn btn-flat back-button" @route={{@route}}>
+  <LinkTo
+    class="btn btn-transparent back-button"
+    @route={{@route}}
+    @models={{if @model (array @model) (array)}}
+  >
     {{icon "chevron-left"}}
     {{i18n (or @label "back_button")}}
   </LinkTo>