fix: preserve negative balances in account displays

[chenyme]

Summary

修复负余额在订阅页面和后台账户管理中错误显示为 0 的问题。

后端账户余额响应现在保留负数；前端使用独立的余额格式化逻辑展示正负余额，并在后台账户列表中遵循管理员配置的 USD/CNY 展示货币。管理员仍不能主动设置负余额，但可以正常编辑已有负余额用户的其他资料。

同时精简账户页的计费配置请求，避免无关的模型或定价接口失败影响余额币种展示。

Fixes #518

Change type

• Bug fix
• Feature
• Documentation
• Refactor
• Configuration / deployment
• Security hardening
• Other

Affected areas

• Frontend / UI
• Backend / API
• Authentication / authorization
• Conversations / streaming
• Files / RAG / extraction
• Model routing / providers
• MCP / tools
• Billing / payments
• Admin console
• Deployment / Docker / configuration
• Documentation

Verification

• cd frontend && pnpm lint
• cd frontend && pnpm typecheck
• cd backend && go test ./internal/transport/http/billing
• pnpm api:check
• git diff --check
• Not run; reason: pnpm build 未运行，本次已完成聚焦静态检查与后端测试。

Screenshots, API examples, or logs

负余额现在会按照配置的展示货币正确呈现，例如：

-$1.250000
-¥9.000000

Configuration, migration, and compatibility notes

• 无数据库迁移或配置项变更。
• API 字段结构保持不变，仅修正 balanceUSD 将负余额错误归零的响应语义。
• 管理员余额写入接口仍以 USD 为单位，并继续拒绝设置负余额。
• 编辑和批量余额输入已明确标注为 USD，避免与 CNY 只读展示混淆。
• Swagger 与生成的 TypeScript API 契约无需更新。

Documentation

• Documentation is not needed for this change.
• Documentation was updated.
• Documentation still needs to be updated.

Security and privacy

• No secrets, tokens, credentials, local config, or personal data are included.
• User data access remains scoped by authenticated user context unless an admin-only path explicitly requires broader access.
• Security-sensitive behavior was reviewed, including billing balance reads and admin balance updates.

Checklist

• I searched existing issues and pull requests.
• Changes are focused and do not include unrelated refactors.
• Tests or static verification were run where practical.
• User-facing behavior, deployment steps, API contracts, or configuration changes are documented.
• Generated artifacts are included only when this project explicitly requires them.
• Caches, build output, .pyc files, .env files, and local storage data are not committed.