FIX: bold sidebar header when admin sidebar is disabled (#26993)

Recently a bug was introduced when the admin sidebar section was made bold.

When the admin sidebar is disabled, we display the original sidebar in the admin panel. In that case, an incorrect CSS rule is executed.
```CSS
.admin-area .sidebar-wrapper {
  background-color: var(--d-sidebar-admin-background);
  .sidebar-section-header-text {
    font-weight: bold;
  }
}
```
Bug in this PR: https://github.com/discourse/discourse/pull/26801

To solve it, a custom CSS class with a panel key was added which will allow granular customisations.

app/assets/javascripts/discourse/app/components/sidebar/api-panels.gjs

@@ -1,9 +1,17 @@
+import Component from "@glimmer/component";
+import { service } from "@ember/service";
 import ApiSections from "./api-sections";
 
-const SidebarApiPanels = <template>
-  <div class="sidebar-sections">
-    <ApiSections @collapsable={{@collapsableSections}} />
-  </div>
-</template>;
+export default class SidebarApiPanels extends Component {
+  @service sidebarState;
 
-export default SidebarApiPanels;
+  get panelCssClass() {
+    return `${this.sidebarState.currentPanel.key}-panel`;
+  }
+
+  <template>
+    <div class="sidebar-sections {{this.panelCssClass}}">
+      <ApiSections @collapsable={{@collapsableSections}} />
+    </div>
+  </template>
+}

app/assets/javascripts/discourse/tests/acceptance/sidebar-plugin-api-test.js

@@ -921,7 +921,7 @@ acceptance("Sidebar - Plugin API", function (needs) {
       api.addSidebarPanel((BaseCustomSidebarPanel) => {
         const AdminSidebarPanel = class extends BaseCustomSidebarPanel {
           get key() {
-            return "admin-panel";
+            return "admin";
           }
 
           get hidden() {
@@ -1004,9 +1004,9 @@ acceptance("Sidebar - Plugin API", function (needs) {
             }
           };
         },
-        "admin-panel"
+        "admin"
       );
-      api.setSidebarPanel("admin-panel");
+      api.setSidebarPanel("admin");
       api.setSeparatedSidebarMode();
     });
 
@@ -1019,12 +1019,14 @@ acceptance("Sidebar - Plugin API", function (needs) {
       "test admin section",
       "displays header with correct text"
     );
+    assert.dom(".admin-panel").exists();
     withPluginApi(PLUGIN_API_VERSION, (api) => {
       api.setSidebarPanel("main-panel");
       api.setCombinedSidebarMode();
     });
     await visit("/");
     assert.dom(".sidebar__panel-switch-button").doesNotExist();
+    assert.dom(".admin-panel").doesNotExist();
     assert
       .dom(".sidebar-section[data-section-name='test-admin-section']")
       .doesNotExist();

app/assets/stylesheets/common/admin/admin_revamp.scss

@@ -26,7 +26,7 @@
   }
 }
 
-.admin-area .sidebar-wrapper {
+.admin-area .sidebar-wrapper .admin-panel {
   background-color: var(--d-sidebar-admin-background);
   .sidebar-section-header-text {
     font-weight: bold;