Update API and WebSocket base URLs to use environment variables for better configuration management

backend/src/main/java/com/music/config/SpaForwardConfig.java

@@ -0,0 +1,34 @@
+package com.music.config;
+
+import org.springframework.context.annotation.Configuration;
+import org.springframework.core.io.Resource;
+import org.springframework.web.servlet.config.annotation.ResourceHandlerRegistry;
+import org.springframework.web.servlet.config.annotation.WebMvcConfigurer;
+import org.springframework.web.servlet.resource.PathResourceResolver;
+
+import java.io.IOException;
+
+/**
+ * 生产环境：前端 SPA 由 Spring Boot 托管时，未匹配到的路径回退到 index.html。
+ */
+@Configuration
+public class SpaForwardConfig implements WebMvcConfigurer {
+
+    @Override
+    public void addResourceHandlers(ResourceHandlerRegistry registry) {
+        registry.addResourceHandler("/**")
+                .addResourceLocations("classpath:/static/")
+                .resourceChain(true)
+                .addResolver(new PathResourceResolver() {
+                    @Override
+                    protected Resource getResource(String resourcePath, Resource location) throws IOException {
+                        Resource resource = location.createRelative(resourcePath);
+                        if (resource.exists() && resource.isReadable()) {
+                            return resource;
+                        }
+                        // SPA 路由回退到 index.html
+                        return location.createRelative("index.html");
+                    }
+                });
+    }
+}