Allow desktop app to connect to selfhosted servers (#4902)

# Description of Changes
Changes the desktop app to allow connections to self-hosted servers on
first startup. This was quite involved and hit loads of CORS issues all
through the stack, but I think it's working now. This also changes the
bundled backend to spawn on an OS-decided port rather than always
spawning on `8080`, which means that the user can have other things
running on port `8080` now and the app will still work fine. There were
quite a few places that needed to be updated to decouple the app from
explicitly using `8080` and I was originally going to split those
changes out into another PR (#4939), but I couldn't get it working
independently in the time I had, so the diff here is just going to be
complex and contian two distinct changes - sorry 🙁

app/core/src/main/java/stirling/software/SPDF/SPDFApplication.java

@@ -12,6 +12,8 @@ import java.util.Properties;
 
 import org.springframework.boot.SpringApplication;
 import org.springframework.boot.autoconfigure.SpringBootApplication;
+import org.springframework.boot.web.context.WebServerInitializedEvent;
+import org.springframework.context.event.EventListener;
 import org.springframework.core.env.Environment;
 import org.springframework.scheduling.annotation.EnableScheduling;
 
@@ -198,6 +200,14 @@ public class SPDFApplication {
         // }
     }
 
+    @EventListener
+    public void onWebServerInitialized(WebServerInitializedEvent event) {
+        int actualPort = event.getWebServer().getPort();
+        serverPortStatic = String.valueOf(actualPort);
+        // Log the actual runtime port for Tauri to parse
+        log.info("Stirling-PDF running on port: {}", actualPort);
+    }
+
     private static void printStartupLogs() {
         log.info("Stirling-PDF Started.");
         String url = baseUrlStatic + ":" + getStaticPort() + contextPathStatic;