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_en.dart

@@ -332,4 +332,59 @@ class StringsEn extends AppText {
   String get ob3Title => 'Private by design';
   @override
   String get ob3Sub => 'End-to-end encrypted. We keep zero browsing logs.';
+
+  @override
+  String get purchaseTitle => 'Purchase';
+  @override
+  String get paymentTitle => 'Payment';
+  @override
+  String get buyNow => 'Buy now';
+  @override
+  String get payMethodAlipay => 'Alipay';
+  @override
+  String get payMethodCrypto => 'USDT (TRC20)';
+  @override
+  String get choosePayMethod => 'Choose payment method';
+  @override
+  String get proMonthly => 'Pro · Monthly';
+  @override
+  String get proQuarterly => 'Pro · Quarterly';
+  @override
+  String get proYearly => 'Pro · Yearly';
+  @override
+  String get perQuarter => '/quarter';
+  @override
+  String get perYear => '/year';
+  @override
+  String get payAmountLabel => 'Amount';
+  @override
+  String get payAddressLabel => 'Address';
+  @override
+  String get payNetworkLabel => 'Network';
+  @override
+  String get payExactAmountHint => 'Send the exact amount; activates automatically';
+  @override
+  String get copied => 'Copied';
+  @override
+  String get openAlipay => 'Pay with Alipay';
+  @override
+  String get openAlipayHint => 'Return after paying; this page refreshes automatically';
+  @override
+  String get awaitingPayment => 'Awaiting payment';
+  @override
+  String get paySucceeded => 'Activated';
+  @override
+  String get payExpiresAt => 'Valid until';
+  @override
+  String get payDone => 'Done';
+  @override
+  String get payFailed => 'Payment failed';
+  @override
+  String get payRetry => 'Retry';
+  @override
+  String get switchPayMethod => 'Switch payment method';
+  @override
+  String get cancelOrder => 'Cancel order';
+  @override
+  String get qrNotSupported => 'Copy and open in Alipay';
 }