Remove backend UI (#4023)

# Description of Changes

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- Why the change was made
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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 <[email protected]>

app/proprietary/build.gradle

@@ -35,6 +35,8 @@ dependencies {
     api 'org.springframework:spring-webmvc'
     api 'org.springframework.session:spring-session-core'
     api "org.springframework.security:spring-security-core:$springSecuritySamlVersion"
+    api "org.springframework.security:spring-security-web:$springSecuritySamlVersion"
+    api "org.springframework.security:spring-security-config:$springSecuritySamlVersion"
     api "org.springframework.security:spring-security-saml2-service-provider:$springSecuritySamlVersion"
     api 'org.springframework.boot:spring-boot-starter-jetty'
     api 'org.springframework.boot:spring-boot-starter-security'
@@ -47,7 +49,7 @@ dependencies {
     // https://mvnrepository.com/artifact/com.bucket4j/bucket4j_jdk17
     implementation 'org.bouncycastle:bcprov-jdk18on:1.81'
 
-    implementation 'org.thymeleaf.extras:thymeleaf-extras-springsecurity5:3.1.3.RELEASE'
+    // implementation 'org.thymeleaf.extras:thymeleaf-extras-springsecurity5:3.1.3.RELEASE' // Removed - UI moved to React frontend
     api 'io.micrometer:micrometer-registry-prometheus'
     implementation 'com.unboundid.product.scim2:scim2-sdk-client:4.0.0'
     runtimeOnly 'com.h2database:h2:2.3.232' // Don't upgrade h2database