fix: 修复未登录皮肤站时启动卡住且不再弹玩家选择

未登录皮肤站时启动游戏会弹窗要求选择/登录玩家。若点击“登录皮肤站”
跳转后不登录直接返回，再次启动不会重新弹窗，而是卡在“正在启动”。

原因：InstancePlayerModal 的 signInSkinSite 跳转登录时只隐藏弹窗，
没有 settle 玩家选择的 Promise，因此 prepareInstancePlayer 在
preparing 表中留下的任务永不结束。再次启动时复用了这个悬空任务，
既不再弹窗也无法继续启动。

修复：
- instance-player 新增哨兵错误 PlayerSelectionNavigatedAwayError。
- signInSkinSite 跳转登录前以该错误 reject 当前选择，使
  prepareInstancePlayer 的任务正常结束、清理 in-flight 记录，下次
  启动会重新弹出玩家选择。
- App.vue 的启动错误处理识别该哨兵：跳转登录是用户主动操作而非启动
  失败，静默中止启动，不弹错误提示。

apps/app-frontend/src/App.vue

@@ -113,6 +113,7 @@ import {
 } from '@/helpers/events.js'
 import { install_create_modpack_instance, install_get_modpack_preview } from '@/helpers/install'
 import { type DirectLinkSyncReport, get as getInstance, run } from '@/helpers/instance'
+import { PlayerSelectionNavigatedAwayError } from '@/helpers/instance-player'
 import { reconcileMojangAuthSourceAtStartup } from '@/helpers/mojang-auth'
 import { cancelLogin, get as getCreds, login, logout } from '@/helpers/mr_auth.ts'
 import { mergeUrlQuery, parseModrinthLink } from '@/helpers/project-links.ts'
@@ -1757,6 +1758,10 @@ async function handleCommand(e) {
 	} else if (e.event === 'LaunchInstance') {
 		const instance = await getInstance(e.id).catch(() => null)
 		const handleLaunchCommandError = async (launchError) => {
+			// Navigating to the skin-site login to pick a player is a deliberate
+			// user action, not a launch failure: stay silent and let the user
+			// re-trigger the launch after signing in.
+			if (launchError instanceof PlayerSelectionNavigatedAwayError) return
 			const handled =
 				(await minecraftCrashModal.value?.handleLaunchError(launchError, {
 					instance_id: e.id,

apps/app-frontend/src/components/instance/InstancePlayerModal.vue

@@ -14,6 +14,7 @@ import {
 import { users } from '@/helpers/auth'
 import { getInstanceMode } from '@/helpers/hosted-packs'
 import {
+	PlayerSelectionNavigatedAwayError,
 	registerInstancePlayerPicker,
 	saveInstancePlayer,
 	waitForSkinSiteSession,
@@ -127,6 +128,14 @@ async function select(player: PlayerChoice) {
 function signInSkinSite() {
 	awaitingSkinLogin.value = true
 	hideForLogin = true
+	active.value = false
+	generation++
+	// Settle the pending selection so `prepareInstancePlayer` releases its
+	// in-flight entry; otherwise a later launch would await this forever and
+	// never re-prompt. The caller treats this sentinel as a silent abort.
+	rejectSelection?.(new PlayerSelectionNavigatedAwayError())
+	resolveSelection = undefined
+	rejectSelection = undefined
 	openSkinSiteLogin()
 	modal.value?.hide()
 	void router.push('/starlight-skin')

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

@@ -17,6 +17,18 @@ export type InstancePlayer = {
 	skin_site_user?: string | null
 }
 export type PlayerChoice = InstancePlayer & { head?: string }
+
+/**
+ * Thrown when the player picker is dismissed because the user navigated to the
+ * skin-site login page. Callers should treat this as a silent launch abort
+ * (the user will pick a player next time), not as a real failure.
+ */
+export class PlayerSelectionNavigatedAwayError extends Error {
+	constructor() {
+		super('已跳转至皮肤站登录，请登录后重新启动。')
+		this.name = 'PlayerSelectionNavigatedAwayError'
+	}
+}
 type Picker = (instanceId: string, locked: InstancePlayer | null) => Promise<InstancePlayer>
 let picker: Picker | undefined
 const preparing = new Map<string, Promise<void>>()