Update multi-toolAdvert to respect SYSTEM_ROOTURIPATH. (#3776)

Closes #3775

Currently, the advert link assumes `SYSTEM_ROOTURIPATH` to be `/`. As
described in the issue, this isn't always the case. Simply removing `/`
fixes the issue by taking the same approach to endpoints as
`navbarEntry`.

---

## Checklist

### General

- [x] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [x] I have read the [Stirling-PDF Developer
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/DeveloperGuide.md)
(if applicable)
- [ ] I have read the [How to add new languages to
Stirling-PDF](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/HowToAddNewLanguage.md)
(if applicable)
- [x] I have performed a self-review of my own code
- [x] My changes generate no new warnings

### Documentation

- [ ] I have updated relevant docs on [Stirling-PDF's doc
repo](https://github.com/Stirling-Tools/Stirling-Tools.github.io/blob/main/docs/)
(if functionality has heavily changed)
- [ ] I have read the section [Add New Translation
Tags](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/HowToAddNewLanguage.md#add-new-translation-tags)
(for new translation tags only)

### UI Changes (if applicable)

- [ ] Screenshots or videos demonstrating the UI changes are attached
(e.g., as comments or direct attachments in the PR)

### Testing (if applicable)

- [ ] I have tested my changes locally. Refer to the [Testing
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/DeveloperGuide.md#6-testing)
for more details.

stirling-pdf/src/main/resources/templates/fragments/multi-toolAdvert.html

@@ -1,7 +1,7 @@
 <div th:fragment="multi-toolAdvert" class="mx-auto">
   <div id="multi-toolAdvert" class="multi-toolAdvert">
     <div>
-      <span th:utext="#{multiTool-advert.message(|/multi-tool|)}"></span>
+      <span th:utext="#{multiTool-advert.message(|multi-tool|)}"></span>
       <button id="closeMultiToolAdvert" style="position: absolute;
   inset-inline-end: 12px;
   inset-block-start: 10px;
@@ -62,4 +62,4 @@
       }
     });
   </script>
-</div>
+</div>