feat(client): 购买页三档 + 支付页 render_type 多态(crypto/redirect/qr 预留)+ 导航接线

购买页三档 pro(月/季/年,金额来自 GET /v1/pay/catalog)选档 → 选支付方式 → 下单;
支付页按 session.render_type 多态渲染(crypto_address 地址+精确金额+复制、redirect
外链拉起、qr 预留复制兜底),轮询用 Task 6 的 paymentFlowProvider,409
CURRENCY_MISMATCH 走「换方式开新单」。同时收口 paymentFlowProvider 非 autoDispose
带来的轮询生命周期缺口——支付页 dispose 时停轮询(cancel() 延后到微任务执行,避免
在自身 unmount 期间同步改 state 炸 Riverpod 断言)。

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_013nMthbVEmQquxBRKb9Fj8u

client/lib/l10n/strings_zh.dart

@@ -326,4 +326,59 @@ class StringsZh extends AppText {
   String get ob3Title => '安全 · 无日志';
   @override
   String get ob3Sub => '端到端加密,我们不记录你的任何浏览数据。';
+
+  @override
+  String get purchaseTitle => '购买套餐';
+  @override
+  String get paymentTitle => '支付';
+  @override
+  String get buyNow => '立即购买';
+  @override
+  String get payMethodAlipay => '支付宝';
+  @override
+  String get payMethodCrypto => 'USDT (TRC20)';
+  @override
+  String get choosePayMethod => '选择支付方式';
+  @override
+  String get proMonthly => '专业版·月付';
+  @override
+  String get proQuarterly => '专业版·季付';
+  @override
+  String get proYearly => '专业版·年付';
+  @override
+  String get perQuarter => '/季';
+  @override
+  String get perYear => '/年';
+  @override
+  String get payAmountLabel => '转账金额';
+  @override
+  String get payAddressLabel => '收款地址';
+  @override
+  String get payNetworkLabel => '网络';
+  @override
+  String get payExactAmountHint => '金额须精确一致，到账后自动开通';
+  @override
+  String get copied => '已复制';
+  @override
+  String get openAlipay => '打开支付宝支付';
+  @override
+  String get openAlipayHint => '完成支付后返回，本页会自动刷新';
+  @override
+  String get awaitingPayment => '等待付款';
+  @override
+  String get paySucceeded => '已开通';
+  @override
+  String get payExpiresAt => '有效期至';
+  @override
+  String get payDone => '完成';
+  @override
+  String get payFailed => '支付失败';
+  @override
+  String get payRetry => '重试';
+  @override
+  String get switchPayMethod => '换一种支付方式';
+  @override
+  String get cancelOrder => '取消订单';
+  @override
+  String get qrNotSupported => '请复制内容后在支付宝内打开';
 }