fix: 登录皮肤站后侧栏立即显示玩家选择器

登录 StarLight 皮肤站后，侧栏账户选择器（AccountsCard）不会列出
皮肤站玩家，必须先使用该玩家启动一次实例，玩家才会出现。

原因：账户选择器列出的是 auth::get_users 的账户库，而皮肤站玩家
是 iframe 推送的另一套数据。两者唯一的打通点是
auth|login_skin_site_player（把玩家注册为 yggdrasil 账户），而它
此前只在启动实例选择玩家时被调用。

修复：
- instance-player 新增 registerSkinSitePlayers：玩家列表就绪后，
  把尚未注册的皮肤站玩家逐个注册为账户。幂等（跳过账户库中已存在
  的 profile id）、best-effort（单个失败不影响其余）、每个玩家
  单独获取下载 token（皮肤站登录接口可能将 token 绑定到单一玩家）。
- AccountsCard 的玩家监听新增 skinSiteUser 依赖，玩家就绪后调用
  注册并刷新账户列表，使玩家无需先启动实例即可在选择器中选择。

apps/app-frontend/src/helpers/instance-player.ts

@@ -56,6 +56,52 @@ export async function authenticateInstancePlayer(player: InstancePlayer) {
 	}
 }
 
+/**
+ * Registers every skin-site player as a launcher account so they show up in the
+ * account picker immediately after signing in to the skin site, instead of only
+ * after a first launch. Idempotent: players that already exist in the account
+ * list (matched by profile UUID) are skipped, and already-signed-in players are
+ * not re-requested. Best-effort: a single player failing does not abort the rest.
+ */
+export async function registerSkinSitePlayers(
+	playerIds: string[],
+	userUuid: string,
+): Promise<void> {
+	if (playerIds.length === 0) return
+	if (skinSiteStatus.value !== 'signed-in' || skinSiteUser.value?.uuid !== userUuid) return
+
+	let known = new Set<string>()
+	try {
+		const existing = await users()
+		known = new Set(
+			(existing as Array<{ profile?: { id?: string } }>)
+				.map((account) => account?.profile?.id)
+				.filter((id): id is string => typeof id === 'string'),
+		)
+	} catch {
+		// If the account list cannot be read, still attempt to register; the
+		// backend upsert is idempotent.
+	}
+
+	for (const playerId of playerIds) {
+		if (known.has(playerId)) continue
+		if (skinSiteStatus.value !== 'signed-in' || skinSiteUser.value?.uuid !== userUuid) return
+		try {
+			// Request a fresh download token per player: the skin site login
+			// endpoint may bind a token to a single player id.
+			const token = await requestSkinSiteDownloadToken()
+			await invoke('plugin:auth|login_skin_site_player', {
+				token,
+				playerId,
+				userId: userUuid,
+			})
+			known.add(playerId)
+		} catch (error) {
+			console.warn(`Failed to register skin site player ${playerId}:`, error)
+		}
+	}
+}
+
 export async function saveInstancePlayer(instanceId: string, player: InstancePlayer) {
 	await authenticateInstancePlayer(player)
 	await invoke('plugin:auth|set_instance_player', { instanceId, player })