API mode UI (#5287)

# Description of Changes


<img width="1945" height="1895" alt="image"
src="https://github.com/user-attachments/assets/4ace2535-d0ec-44a7-b57c-df9bcdd5c5d7"
/>

---

## 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)

### Translations (if applicable)

- [ ] I ran
[`scripts/counter_translation.py`](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/docs/counter_translation.md)

### 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.

app/core/build.gradle

@@ -268,6 +268,18 @@ tasks.register('cleanFrontendAssets', Delete) {
     delete generatedFrontendPaths.collect { new File(resourcesStaticDir, it) }
 }
 
+tasks.register('copyApiLandingPage', Copy) {
+    group = 'frontend'
+    description = 'Copy API landing page to index.html for backend-only mode'
+    from(new File(resourcesStaticDir, 'api-landing.html'))
+    into(resourcesStaticDir)
+    rename('api-landing.html', 'index.html')
+    dependsOn cleanFrontendAssets
+    doFirst {
+        println "Copying API landing page to index.html for backend-only mode..."
+    }
+}
+
 // Ensure copyFrontendAssets runs after spotless tasks
 tasks.named('copyFrontendAssets').configure {
     mustRunAfter tasks.matching { it.name.startsWith('spotless') }
@@ -277,9 +289,9 @@ if (buildWithFrontend) {
     println "Frontend build enabled - JAR will include React frontend"
     processResources.dependsOn copyFrontendAssets
 } else {
-    println "Frontend build disabled - JAR will be backend-only"
-    // When not building the UI, ensure any stale frontend assets are removed
-    processResources.dependsOn cleanFrontendAssets
+    println "Frontend build disabled - JAR will be backend-only with API landing page"
+    // When not building the UI, ensure any stale frontend assets are removed and use API landing page
+    processResources.dependsOn copyApiLandingPage
 }
 
 bootJar.dependsOn ':common:jar'