SSO Refactoring (#2818)

# Description of Changes

* Refactoring of SSO code around OAuth & SAML 2
* Enabling auto-login with SAML 2 via the new `SSOAutoLogin` property
* Correcting typos & general cleanup

---

## 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)
- [x] 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

- [x] 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)

- [x] 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.

src/main/resources/templates/login.html

@@ -42,7 +42,7 @@
               const runningEE = /*[[${@runningEE}]]*/ false;
               const SSOAutoLogin = /*[[${@SSOAutoLogin}]]*/ false;
               const loginMethod = /*[[${loginMethod}]]*/ 'normal';
-              const providerList = /*[[${providerlist}]]*/ {};
+              const providerList = /*[[${providerList}]]*/ {};
               const shouldAutoRedirect = !hasRedirectError &&
                   !hasLogout &&
                   !hasMessage &&
@@ -98,14 +98,14 @@
           <img class="my-4" th:src="@{'/favicon.svg'}" alt="favicon" width="144" height="144">
 
           <h1 class="h1 mb-3 fw-normal" th:text="${@appName}">Stirling-PDF</h1>
-          <div th:if="${altLogin} and (${loginMethod} == 'all' or ${loginMethod} == 'oauth2')">
+          <div th:if="${altLogin} and (${loginMethod} == 'all' or ${loginMethod} == 'oauth2' or ${loginMethod} == 'saml2')">
             <a href="#" class="w-100 btn btn-lg btn-primary" data-bs-toggle="modal" data-bs-target="#loginsModal" th:text="#{login.ssoSignIn}">Login Via SSO</a>
             <br>
             <br>
             <hr />
           </div>
-          <div th:if="${erroroauth}" class="alert alert-danger text-center">
-            <div th:if="${erroroauth}" th:text="#{${erroroauth}}">OAuth2: Error Message</div>
+          <div th:if="${errorOAuth}" class="alert alert-danger text-center">
+            <div th:if="${errorOAuth}" th:text="#{${errorOAuth}}">OAuth2: Error Message</div>
           </div>
 
           <div th:if="${error}" class="alert alert-danger text-center">
@@ -164,7 +164,7 @@
             </button>
           </div>
           <div class="modal-body">
-            <div class="mb-3" th:each="provider : ${providerlist}">
+            <div class="mb-3" th:each="provider : ${providerList}">
               <a th:href="@{|${provider.key}|}" th:text="${provider.value}" class="w-100 btn btn-lg btn-primary">Login Provider</a>
             </div>
           </div>