UX: Fix blank space for discourse-subscriptions plugin on user-main-nav (#36408)

The original discourse-subscriptions plugin uses a condition rendering
logic to make sure the button doesn't exist on other users' main page.

However, the logic doesn't consider the parent node `<li id="ember136"
class="user-main-nav-outlet billing ember-view">`. As a matter of fact,
there exists a blank that takes space and make the nav-bar misaligned.

This PR change the condition rendering logic `{{#if this.viewingSelf}}`
to the class `@classNameBindings("viewingSelf::hidden")`, so that the
whole element can hide entirely without leaving a blank space.

Before:
<img width="918" height="750" alt="image"
src="https://github.com/user-attachments/assets/79cf0886-a77c-467c-a00c-b06a3ae3a2eb"
/>

After:
<img width="982" height="837" alt="image"
src="https://github.com/user-attachments/assets/cea5139c-ffff-4a99-a1fb-b537ebc98c94"
/>

plugins/discourse-subscriptions/assets/javascripts/discourse/connectors/user-main-nav/billing.gjs

@@ -2,12 +2,17 @@
 import Component from "@ember/component";
 import { LinkTo } from "@ember/routing";
 import { service } from "@ember/service";
-import { classNames, tagName } from "@ember-decorators/component";
+import {
+  classNameBindings,
+  classNames,
+  tagName,
+} from "@ember-decorators/component";
 import icon from "discourse/helpers/d-icon";
 import { i18n } from "discourse-i18n";
 
 @tagName("li")
 @classNames("user-main-nav-outlet", "billing")
+@classNameBindings("viewingSelf::hidden")
 export default class Billing extends Component {
   @service currentUser;
 
@@ -20,11 +25,9 @@ export default class Billing extends Component {
   }
 
   <template>
-    {{#if this.viewingSelf}}
-      <LinkTo @route="user.billing">
-        {{icon "far-credit-card"}}
-        <span>{{i18n "discourse_subscriptions.navigation.billing"}}</span>
-      </LinkTo>
-    {{/if}}
+    <LinkTo @route="user.billing">
+      {{icon "far-credit-card"}}
+      <span>{{i18n "discourse_subscriptions.navigation.billing"}}</span>
+    </LinkTo>
   </template>
 }