fix: tab名称改为余额充值/套餐订阅、有效期直接显示配置值、模型标签跟随平台颜色

1. MainTabs: "按量付费"→"余额充值"，"包月套餐"→"套餐订阅"
2. subscription-utils: 去掉所有智能转换，配置什么就显示什么(1月/30天/2周)
3. platform-style: 新增 modelTag 样式字段，每个平台独立颜色
4. ChannelCard: 模型标签颜色跟随平台(橙/绿/蓝/粉/紫)而非统一蓝色

src/components/MainTabs.tsx

@@ -18,9 +18,9 @@ export default function MainTabs({ activeTab, onTabChange, showSubscribeTab, sho
 
   const tabs: { key: 'topup' | 'subscribe'; label: string }[] = [];
   if (showTopUpTab) {
-    tabs.push({ key: 'topup', label: pickLocaleText(locale, '按量付费', 'Pay-as-you-go') });
+    tabs.push({ key: 'topup', label: pickLocaleText(locale, '余额充值', 'Top Up') });
   }
-  tabs.push({ key: 'subscribe', label: pickLocaleText(locale, '包月套餐', 'Subscription') });
+  tabs.push({ key: 'subscribe', label: pickLocaleText(locale, '套餐订阅', 'Subscription') });
 
   // 只有一个 tab 时不显示切换器
   if (tabs.length <= 1) return null;