feat: persist offline player name in LinkEntry for proper display in links command

Add offlinePlayerName field to LinkEntry record, stored only for
offline-mode UUIDs (version 3) since Mojang API cannot resolve them.
Online-mode players continue to use real-time Mojang API resolution.
The field is omitted from JSON when null for clean serialization.

MojangUtils.resolvePlayerName() now accepts an optional fallback name
for offline UUIDs, returning "N/A" when no name is available.

Updated README.md storage constraints and query display rules to
reflect the new offline player name persistence.

Co-authored-by: Xujiayao <58985541+Xujiayao@users.noreply.github.com>

README.md

@@ -62,8 +62,9 @@ DMCC 所有运行模式都基于一个统一的通信模型，该模型包含两
 - **一个 Discord 账户可关联多个 Minecraft 账户**（方便玩家管理大号与小号）。
 - **一个 Minecraft 账户只能关联一个 Discord 账户**（确保游戏内身份的绝对唯一性）。
 - **数据持久化**: 绑定关系作为永久数据存储在 `Server` 端的 `account_linking/links.json` 中，以 Discord ID 为主键。
-- **存储约束**: `links.json` 中仅存储绑定关系最小必要字段（Discord ID、Minecraft UUID、添加时间），**不得额外存储**
-  Discord 用户名或 Minecraft 玩家名；显示名称在查询时实时解析。
+- **存储约束**: `links.json` 中存储绑定关系最小必要字段（Discord ID、Minecraft UUID、添加时间）。对于离线模式玩家，
+  额外存储 `offlinePlayerName`（绑定时的玩家名），因为离线 UUID 无法通过 Mojang API 反查玩家名。
+  在线模式玩家不存储玩家名，显示名称在查询时通过 Mojang API 实时解析。Discord 用户名始终不存储，查询时实时解析。
 
 ### 4.2 安全绑定工作流 (严格的 MC 优先原则)
 
@@ -82,7 +83,8 @@ DMCC 所有运行模式都基于一个统一的通信模型，该模型包含两
 
 - **Discord `/unlink`**: 直接取消该 Discord 用户名下的所有 Minecraft 绑定（无需二次确认）。
 - **Minecraft `/dmcc unlink`**: 直接取消“当前执行玩家”对应的绑定关系（无需二次确认）。
-- **查询展示规则**: `links` 查询时实时解析显示名称，若无法解析则回退显示 UUID / Discord ID。
+- **查询展示规则**: `links` 查询时实时解析显示名称；在线模式玩家通过 Mojang API 解析，离线模式玩家使用绑定时记录的玩家名。
+  若无法解析则显示 "N/A"（Minecraft 玩家名）或 Discord ID。
 
 ### 4.4 同步与跨平台交互
 

core/src/main/java/com/xujiayao/discord_mc_chat/commands/impl/LinksCommand.java

@@ -71,7 +71,7 @@ public class LinksCommand implements Command {
 
 			for (LinkedAccountManager.LinkEntry link : links) {
 				String time = DATE_FORMATTER.format(Instant.ofEpochMilli(link.linkedAt()));
-				String mcName = MojangUtils.resolvePlayerName(link.minecraftUuid());
+				String mcName = MojangUtils.resolvePlayerName(link.minecraftUuid(), link.offlinePlayerName());
 				builder.append("\n  - MC: ").append(mcName).append(" (").append(link.minecraftUuid()).append(")");
 				builder.append(" (").append(time).append(")");
 			}

core/src/main/java/com/xujiayao/discord_mc_chat/server/linking/LinkedAccountManager.java

@@ -1,5 +1,6 @@
 package com.xujiayao.discord_mc_chat.server.linking;
 
+import com.fasterxml.jackson.annotation.JsonInclude;
 import com.fasterxml.jackson.core.type.TypeReference;
 import com.xujiayao.discord_mc_chat.utils.i18n.I18nManager;
 
@@ -11,6 +12,7 @@ import java.util.ArrayList;
 import java.util.Collections;
 import java.util.List;
 import java.util.Map;
+import java.util.UUID;
 import java.util.concurrent.ConcurrentHashMap;
 import java.util.function.Function;
 
@@ -151,7 +153,7 @@ public class LinkedAccountManager {
 		}
 
 		LINKED_ACCOUNTS.computeIfAbsent(discordId, k -> new ArrayList<>())
-				.add(new LinkEntry(minecraftUuid, System.currentTimeMillis()));
+				.add(new LinkEntry(minecraftUuid, System.currentTimeMillis(), isOfflineUuid(minecraftUuid) ? minecraftName : null));
 		UUID_TO_DISCORD.put(minecraftUuid, discordId);
 
 		LOGGER.info(I18nManager.getDmccTranslation("linking.manager.linked", discordName, discordId, minecraftName, minecraftUuid));
@@ -260,12 +262,32 @@ public class LinkedAccountManager {
 		return Collections.unmodifiableMap(LINKED_ACCOUNTS);
 	}
 
+	/**
+	 * Checks if a Minecraft UUID is an offline-mode UUID (version 3).
+	 *
+	 * @param uuidString The UUID string.
+	 * @return true if the UUID is offline-mode (version 3), false otherwise.
+	 */
+	private static boolean isOfflineUuid(String uuidString) {
+		try {
+			return UUID.fromString(uuidString).version() == 3;
+		} catch (Exception e) {
+			return false;
+		}
+	}
+
 	/**
 	 * A linked Minecraft account entry.
 	 *
-	 * @param minecraftUuid The UUID of the linked Minecraft account.
-	 * @param linkedAt      The timestamp (epoch millis) when the link was created.
+	 * @param minecraftUuid     The UUID of the linked Minecraft account.
+	 * @param linkedAt          The timestamp (epoch millis) when the link was created.
+	 * @param offlinePlayerName The player name stored at link time for offline-mode UUIDs only.
+	 *                          {@code null} for online-mode players (resolvable via Mojang API).
 	 */
-	public record LinkEntry(String minecraftUuid, long linkedAt) {
+	public record LinkEntry(
+			String minecraftUuid,
+			long linkedAt,
+			@JsonInclude(JsonInclude.Include.NON_NULL) String offlinePlayerName
+	) {
 	}
 }

core/src/main/java/com/xujiayao/discord_mc_chat/utils/MojangUtils.java

@@ -33,6 +33,22 @@ public class MojangUtils {
 	 * @return The resolved player name, or the original UUID string if resolution fails.
 	 */
 	public static String resolvePlayerName(String uuidString) {
+		return resolvePlayerName(uuidString, null);
+	}
+
+	/**
+	 * Resolves a Minecraft player name from a UUID string, with an optional fallback name
+	 * for offline-mode UUIDs.
+	 * <p>
+	 * If the UUID is an offline-mode UUID (version 3), {@code offlineFallbackName} is returned
+	 * if non-null; otherwise "N/A" is returned. For online-mode UUIDs (version 4), the Mojang
+	 * session server is queried. Network failures fall back to the raw UUID.
+	 *
+	 * @param uuidString          The UUID string (standard dashed format).
+	 * @param offlineFallbackName The player name to use for offline UUIDs, or null.
+	 * @return The resolved player name, or the fallback/"N/A"/UUID string if resolution fails.
+	 */
+	public static String resolvePlayerName(String uuidString, String offlineFallbackName) {
 		String cached = NAME_CACHE.get(uuidString);
 		if (cached != null) {
 			return cached;
@@ -44,8 +60,10 @@ public class MojangUtils {
 			// Check if this is an offline-mode UUID (version 3)
 			if (uuid.version() == 3) {
 				// Offline UUIDs are generated from "OfflinePlayer:" + name
-				// We cannot reverse this, so return the raw UUID
-				return uuidString;
+				// We cannot reverse this, so use the fallback name or "N/A"
+				String name = (offlineFallbackName != null) ? offlineFallbackName : "N/A";
+				NAME_CACHE.put(uuidString, name);
+				return name;
 			}
 
 			// Online UUID (version 4) - query Mojang