feat(client): pay v2 支付渠道 switch + 订单列表/详情端到端

购买/支付(Phase B):
- 新 SegSwitch 段控 widget(镜像 .segswitch 原子)
- 购买页重构:顶部支付渠道 switch(默认 USDT→加密货币 / 人民币→支付宝),套餐价随渠道分币种显示,删底部弹窗选方式;点购买直达支付页
- 支付页加订单信息卡(套餐/渠道/金额按币种)
- models:PayChannel + PayCatalogItem.priceUsdtMicro/priceLabel(ch)/perMonthLabel(ch)

订单(P0,全新):
- payment_api.listOrders() + orders_provider(列表 + 详情 family)
- OrdersScreen 列表→详情(状态 pill / 可复制订单号 / 继续支付·重新购买)
- 导航:NavView.orders + desktop_shell 分支 + account 入口

配套:
- l10n +19 getter(支付渠道 4 + 订单 15)×6 语言,l10n 闸绿
- 图标单源迁移 lucide_icons_flutter + pangolin_icons.dart(codegen)
- 联系渠道订正(Telegram/邮件 support@yanmeiai.com/官网) + 前端去广告&时长
- 测试:购买 switch 分币种 + 订单列表/空态/详情 + 假 API listOrders;golden 重录
- flutter analyze 0 error,flutter test 226 全绿

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013nMthbVEmQquxBRKb9Fj8u

client/lib/state/connection_provider.dart

@@ -41,7 +41,6 @@ class ConnectionState {
     required this.phase,
     this.elapsed = Duration.zero,
     this.error,
-    this.freeCountdown,
   });
 
   final VpnPhase phase;
@@ -50,15 +49,10 @@ class ConnectionState {
   /// 连接失败/中断原因(已本地化);null = 无错误。供 UI 提示,不再静默吞掉。
   final String? error;
 
-  /// 免费版连接期剩余额度(倒计时);null = 不适用(会员/未连接/额度不限)。
-  /// 由状态机按「连接时锁定的剩余额度 − 已用时长」本地推算,归零即自动切断。
-  final Duration? freeCountdown;
-
-  ConnectionState copyWith({VpnPhase? phase, Duration? elapsed, Duration? freeCountdown}) =>
+  ConnectionState copyWith({VpnPhase? phase, Duration? elapsed}) =>
       ConnectionState(
         phase: phase ?? this.phase,
         elapsed: elapsed ?? this.elapsed,
-        freeCountdown: freeCountdown ?? this.freeCountdown,
       );
 
   @override
@@ -66,11 +60,10 @@ class ConnectionState {
       other is ConnectionState &&
       other.phase == phase &&
       other.elapsed == elapsed &&
-      other.error == error &&
-      other.freeCountdown == freeCountdown;
+      other.error == error;
 
   @override
-  int get hashCode => Object.hash(phase, elapsed, error, freeCountdown);
+  int get hashCode => Object.hash(phase, elapsed, error);
 }
 
 // ── 连通看门狗 ─────────────────────────────────────────────────────
@@ -503,17 +496,14 @@ class ConnectionController extends StateNotifier<ConnectionState> {
     if (!mounted || state.phase != VpnPhase.on || at == null) return;
     final elapsed = _now().difference(at);
 
-    Duration? countdown;
+    // 免费版:到点(锁定额度 − 已用秒 ≤ 0)即自动切断(enforcement,非展示;
+    // 额度 UI 已移除,不再推算展示用倒计时)。null = 会员/额度不限,不切。
     final capSec = _freeRemainingSec;
-    if (capSec != null) {
-      final leftSec = capSec - elapsed.inSeconds;
-      if (leftSec <= 0) {
-        unawaited(_onFreeQuotaExhausted());
-        return;
-      }
-      countdown = Duration(seconds: leftSec);
+    if (capSec != null && capSec - elapsed.inSeconds <= 0) {
+      unawaited(_onFreeQuotaExhausted());
+      return;
     }
-    state = ConnectionState(phase: VpnPhase.on, elapsed: elapsed, freeCountdown: countdown);
+    state = ConnectionState(phase: VpnPhase.on, elapsed: elapsed);
   }
 
   /// 免费额度耗尽:主动切断隧道(不报节点异常),本地置耗尽让按钮灰化,并拉 me 校准。