feat(usage): CodingPlan.site 自动识别 NewAPI + 账单查询体验修复

- detect_provider 新增 codingplan.site → balance:newapi（该站为
  NewAPI 实例，实测确认），保存补齐 TS 侧缺失的 balance:newapi 断言
- 凭据缺失等确定性错误改走 Ok(failure)（原 Err 被前端一律显示为
  「网络异常」），错误消息按 kind 前缀包裹后仍可本地化映射
- formatAmount 收口到 usage-display 统一复用：¤ 占位符号不再渲染、
  金额统一两位小数（配置弹窗此前直接拼原始数值）

src/config/providerPresets.ts

@@ -14,6 +14,7 @@ export type UsageKind =
   | "balance:stepfun"
   | "balance:novita"
   | "balance:kimi"
+  | "balance:newapi"
   | "plan:kimi_coding"
   | "plan:zhipu"
   | "plan:minimax"
@@ -30,6 +31,7 @@ export const SUPPORTED_USAGE_KINDS: ReadonlySet<string> = new Set<UsageKind>([
   "balance:stepfun",
   "balance:novita",
   "balance:kimi",
+  "balance:newapi",
   "plan:kimi_coding",
   "plan:zhipu",
   "plan:minimax",