Saml fixes (#5256)

# Description of Changes

<!--
Please provide a summary of the changes, including:

- What was changed
- Why the change was made
- Any challenges encountered

Closes #(issue_number)
-->

---

## Checklist

### General

- [ ] I have read the [Contribution
Guidelines](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/CONTRIBUTING.md)
- [ ] I have read the [Stirling-PDF Developer
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/devGuide/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/devGuide/HowToAddNewLanguage.md)
(if applicable)
- [ ] I have performed a self-review of my own code
- [ ] 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/devGuide/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/devGuide/DeveloperGuide.md#6-testing)
for more details.

---------

Co-authored-by: ConnorYoh <40631091+ConnorYoh@users.noreply.github.com>

app/common/src/main/java/stirling/software/common/configuration/AppConfig.java

@@ -37,10 +37,6 @@ public class AppConfig {
 
     private final ApplicationProperties applicationProperties;
 
-    @Getter
-    @Value("${baseUrl:http://localhost}")
-    private String baseUrl;
-
     @Getter
     @Value("${server.servlet.context-path:/}")
     private String contextPath;
@@ -49,6 +45,17 @@ public class AppConfig {
     @Value("${server.port:8080}")
     private String serverPort;
 
+    /**
+     * Get the backend URL from system configuration. Falls back to http://localhost if not
+     * configured.
+     *
+     * @return The backend base URL for SAML/OAuth/API callbacks
+     */
+    public String getBackendUrl() {
+        String backendUrl = applicationProperties.getSystem().getBackendUrl();
+        return (backendUrl != null && !backendUrl.isBlank()) ? backendUrl : "http://localhost";
+    }
+
     @Value("${v2}")
     public boolean v2Enabled;