fix: DNS 用 sing-box 1.12+ 新格式 + 引导仅首次显示

- clientconfig/mockserver:客户端配置 DNS 从 legacy address 串格式改为
  type+server 新格式(sing-box 1.13 对 legacy DNS 直接 FATAL,导致连不上)
- 引导页只首次:TokenStore 加 markOnboarded/isOnboarded;main.dart 登录后
  若已引导过直接进主界面,不再每次都弹引导

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

server/internal/httpapi/clientconfig.go

@@ -119,10 +119,12 @@ func BuildClientConfig(node *nodes.NodeRow, dpUUID, deriveKey string) ([]byte, e
 	}
 
 	// DNS: remote over tunnel, local for domestic.
+	// sing-box 1.12+ DNS server format(type+server);旧的 address 串格式在
+	// 1.13 已 FATAL 拒绝(legacy DNS servers deprecated)。
 	dns := map[string]any{
 		"servers": []any{
-			map[string]any{"tag": "remote", "address": "tls://8.8.8.8", "detour": "auto"},
-			map[string]any{"tag": "local", "address": "223.5.5.5", "detour": "direct"},
+			map[string]any{"tag": "remote", "type": "tls", "server": "8.8.8.8", "detour": "auto"},
+			map[string]any{"tag": "local", "type": "udp", "server": "223.5.5.5", "detour": "direct"},
 		},
 		"final":    "remote",
 		"strategy": "ipv4_only",