fix(client): 设置页购买入口改为显示套餐(PurchaseScreen)而非兑换/去哪买链接

- 「购买套餐」→ PurchaseScreen(渠道 switch + 三档套餐 + 下单),不再进 RedeemScreen 的引流卡
- bump 1.0.69+70

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

client/pubspec.yaml

@@ -1,7 +1,7 @@
 name: pangolin_vpn
 description: 穿山甲 · Pangolin — 极简、稳定、跨平台网络加速客户端。
 publish_to: "none"
-version: 1.0.68+69
+version: 1.0.69+70
 
 environment:
   sdk: ^3.5.0