SaaS-aware API landing page (#6585)

# Description of Changes

OLD  (and still current in selfhosted)
<img width="610" height="869" alt="image"
src="https://github.com/user-attachments/assets/f8019298-b4ee-4a68-b928-a9746b64ac1c"
/>


New (in SaaS mode)

<img width="635" height="876" alt="image"
src="https://github.com/user-attachments/assets/6ee4946f-1d7b-42ec-a6f7-75e85739e348"
/>


---

## 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/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 run `task check` to verify linters, typechecks, and tests
pass
- [ ] I have tested my changes locally. Refer to the [Testing
Guide](https://github.com/Stirling-Tools/Stirling-PDF/blob/main/DeveloperGuide.md#7-testing)
for more details.

app/core/src/main/java/stirling/software/SPDF/controller/web/ReactRoutingController.java

@@ -41,6 +41,8 @@ public class ReactRoutingController {
     private boolean indexHtmlExists = false;
     private boolean useExternalIndexHtml = false;
     private boolean loggedMissingIndex = false;
+    private String cachedSaasLandingHtml;
+    private boolean saasLandingExists = false;
 
     @PostConstruct
     public void init() {
@@ -49,6 +51,20 @@ public class ReactRoutingController {
         // Always initialize callback HTML (used for OAuth desktop flow)
         this.cachedCallbackHtml = buildCallbackHtml();
 
+        // SaaS landing page: only present on the classpath when the :saas module is bundled
+        // (app/saas/src/main/resources/static/saas-landing.html). When present it replaces the
+        // root page so the SaaS API host shows its own landing instead of the OSS API-only page.
+        ClassPathResource saasLanding = new ClassPathResource("static/saas-landing.html");
+        if (saasLanding.exists()) {
+            try (InputStream in = saasLanding.getInputStream()) {
+                this.cachedSaasLandingHtml = new String(in.readAllBytes(), StandardCharsets.UTF_8);
+                this.saasLandingExists = true;
+                log.info("SaaS landing page detected; serving it at '/' and '/index.html'");
+            } catch (Exception ex) {
+                log.warn("Failed to read saas-landing.html; falling back to index.html", ex);
+            }
+        }
+
         // Check for external index.html first (customFiles/static/)
         Path externalIndexPath = Paths.get(InstallationPathConfig.getStaticPath(), "index.html");
         log.debug("Checking for custom index.html at: {}", externalIndexPath);
@@ -132,6 +148,18 @@ public class ReactRoutingController {
     @GetMapping(
             value = {"/", "/index.html"},
             produces = MediaType.TEXT_HTML_VALUE)
+    public ResponseEntity<String> serveRootPage(HttpServletRequest request) {
+        // Swap ONLY the root page for SaaS. SPA entry points that delegate to serveIndexHtml
+        // (/auth/callback, /share/{token}, forwarded routes) keep serving the normal shell.
+        if (saasLandingExists && cachedSaasLandingHtml != null) {
+            return ResponseEntity.ok()
+                    .cacheControl(CacheControl.noCache().mustRevalidate())
+                    .contentType(MediaType.TEXT_HTML)
+                    .body(cachedSaasLandingHtml);
+        }
+        return serveIndexHtml(request);
+    }
+
     public ResponseEntity<String> serveIndexHtml(HttpServletRequest request) {
         try {
             if (indexHtmlExists && cachedIndexHtml != null) {