Remove backend UI (#4023)

# 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)
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---

## 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: Copilot <175728472+Copilot@users.noreply.github.com>

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

@@ -10,7 +10,6 @@ import java.util.Properties;
 import java.util.function.Predicate;
 
 import org.springframework.beans.factory.annotation.Value;
-import org.springframework.boot.autoconfigure.condition.ConditionalOnProperty;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.Configuration;
 import org.springframework.context.annotation.Lazy;
@@ -19,9 +18,7 @@ import org.springframework.context.annotation.Scope;
 import org.springframework.core.env.Environment;
 import org.springframework.core.io.ClassPathResource;
 import org.springframework.core.io.Resource;
-import org.springframework.core.io.ResourceLoader;
 import org.springframework.util.ClassUtils;
-import org.thymeleaf.spring6.SpringTemplateEngine;
 
 import lombok.Getter;
 import lombok.RequiredArgsConstructor;
@@ -51,6 +48,7 @@ public class AppConfig {
     @Value("${server.port:8080}")
     private String serverPort;
 
+    /* Commented out Thymeleaf template engine bean - to be removed when frontend migration is complete
     @Bean
     @ConditionalOnProperty(name = "system.customHTMLFiles", havingValue = "true")
     public SpringTemplateEngine templateEngine(ResourceLoader resourceLoader) {
@@ -58,6 +56,7 @@ public class AppConfig {
         templateEngine.addTemplateResolver(new FileFallbackTemplateResolver(resourceLoader));
         return templateEngine;
     }
+    */
 
     @Bean(name = "loginEnabled")
     public boolean loginEnabled() {

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

@@ -1,19 +1,8 @@
 package stirling.software.common.configuration;
 
-import java.io.IOException;
-import java.io.InputStream;
-import java.util.Map;
+/* Commented out entire FileFallbackTemplateResolver class - Thymeleaf dependency removed
+ * This class will be removed when frontend migration to React is complete
 
-import org.springframework.core.io.Resource;
-import org.springframework.core.io.ResourceLoader;
-import org.thymeleaf.IEngineConfiguration;
-import org.thymeleaf.templateresolver.AbstractConfigurableTemplateResolver;
-import org.thymeleaf.templateresource.FileTemplateResource;
-import org.thymeleaf.templateresource.ITemplateResource;
-
-import lombok.extern.slf4j.Slf4j;
-
-import stirling.software.common.model.InputStreamTemplateResource;
 
 @Slf4j
 public class FileFallbackTemplateResolver extends AbstractConfigurableTemplateResolver {
@@ -57,3 +46,4 @@ public class FileFallbackTemplateResolver extends AbstractConfigurableTemplateRe
         return null;
     }
 }
+*/

app/common/src/main/java/stirling/software/common/model/InputStreamTemplateResource.java

@@ -1,11 +1,8 @@
 package stirling.software.common.model;
 
-import java.io.IOException;
-import java.io.InputStream;
-import java.io.InputStreamReader;
-import java.io.Reader;
+/* Commented out entire InputStreamTemplateResource class - Thymeleaf dependency removed
+ * This class will be removed when frontend migration to React is complete
 
-import org.thymeleaf.templateresource.ITemplateResource;
 
 public class InputStreamTemplateResource implements ITemplateResource {
     private InputStream inputStream;
@@ -42,3 +39,4 @@ public class InputStreamTemplateResource implements ITemplateResource {
         return inputStream != null;
     }
 }
+*/