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/components/UsageConfigModal.tsx

@@ -4,7 +4,7 @@ import { invoke } from "@tauri-apps/api/core";
 import { ArrowLeft, Eye, EyeOff, Loader2, Play } from "lucide-react";
 import { useTranslation } from "../i18n";
 import type { TranslationKey } from "../i18n/zh";
-import { localizeUsageError, planLabel } from "../lib/usage-display";
+import { formatAmount, localizeUsageError, planLabel } from "../lib/usage-display";
 import type { Agent, Provider, UsageConfig } from "../types";
 
 interface UsageResult {
@@ -134,7 +134,7 @@ export function UsageConfigModal({ open, agent, provider, onClose, onSave }: Usa
           .map((d) => {
             const parts: string[] = [];
             if (d.planName) parts.push(planLabel(d.planName, t));
-            if (d.remaining != null) parts.push(`${t("usageRemaining")} ${d.remaining}${d.unit ?? ""}`);
+            if (d.remaining != null) parts.push(`${t("usageRemaining")} ${formatAmount(d.remaining, d.unit)}`);
             if (d.used != null && d.total != null && d.total > 0)
               parts.push(`${Math.round((d.used / d.total) * 100)}%`);
             return parts.join(" ");