feat: Improve team management UX with message-based feedback and internal team protection (#3719)

# Description of Changes

- Refactored team management logic to unify and streamline feedback via
`messageType` query parameters.
- Added backend checks to prevent renaming, deleting, or reassigning
users to/from the protected Internal team.
- Updated Thymeleaf templates (`teams.html`, `team-details.html`,
`adminSettings.html`) to support user-visible success and error messages
based on controller redirects.
- Ensured `team.cannotMoveInternalUsers`,
`team.internalTeamNotAccessible`, and `invalidRoleMessage` are properly
internationalized.
- Replaced hardcoded `/adminSettings` redirects with `/teams` for more
consistent UX.

**Why**: 
To provide admins with immediate, meaningful feedback during team
operations and to enforce data integrity around protected teams like
"Internal".

---

## 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)
- [x] 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.

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

proprietary/src/main/resources/templates/accounts/team-details.html

@@ -32,6 +32,11 @@
               </div>
             </div>
 
+            <!-- Alert Messages -->
+            <div th:if="${errorMessage}" class="alert alert-danger data-mb-3">
+              <span th:text="#{${errorMessage}}">Default message if not found</span>
+            </div>
+
             <div class="data-actions data-actions-start">
               <a th:href="@{'/teams'}" class="data-btn data-btn-secondary">
                 <span class="material-symbols-rounded">arrow_back</span>

proprietary/src/main/resources/templates/accounts/teams.html

@@ -29,12 +29,29 @@
           <div class="data-body">
             <!-- Back Button -->
             <div class="data-actions data-actions-start">
-              <a href="/adminSettings" class="data-btn data-btn-secondary">
+              <a th:href="@{'/adminSettings'}" class="data-btn data-btn-secondary">
                 <span class="material-symbols-rounded">arrow_back</span>
                 <span th:text="#{back.toSettings}">Back to Settings</span>
               </a>
             </div>
 
+            <!-- Alert Messages -->
+            <div th:if="${addMessage}" class="alert alert-success data-mb-3">
+              <span th:text="#{${addMessage}}">Default message if not found</span>
+            </div>
+
+            <div th:if="${changeMessage}" class="alert alert-success data-mb-3">
+              <span th:text="#{${changeMessage}}">Default message if not found</span>
+            </div>
+
+            <div th:if="${deleteMessage}" class="alert alert-danger data-mb-3">
+              <span th:text="#{${deleteMessage}}">Default message if not found</span>
+            </div>
+
+            <div th:if="${errorMessage}" class="alert alert-danger data-mb-3">
+              <span th:text="#{${errorMessage}}">Default message if not found</span>
+            </div>
+
             <!-- Create New Team Button -->
             <div class="data-actions">
               <a href="#" th:data-bs-toggle="${@runningProOrHigher} ? 'modal' : null"