feat: use DiceBear for default avatars

src/main/java/com/openisle/service/AvatarGenerator.java

@@ -0,0 +1,25 @@
+package com.openisle.service;
+
+import org.springframework.beans.factory.annotation.Value;
+import org.springframework.stereotype.Service;
+
+import java.net.URLEncoder;
+import java.nio.charset.StandardCharsets;
+
+@Service
+public class AvatarGenerator {
+
+    @Value("${app.avatar.base-url:https://api.dicebear.com/6.x}")
+    private String baseUrl;
+
+    @Value("${app.avatar.style:identicon}")
+    private String style;
+
+    @Value("${app.avatar.size:64}")
+    private int size;
+
+    public String generate(String seed) {
+        String encoded = URLEncoder.encode(seed, StandardCharsets.UTF_8);
+        return String.format("%s/%s/png?seed=%s&size=%d", baseUrl, style, encoded, size);
+    }
+}