Pricing tab shows "No pricing overrides" but provides no UI to add custom model prices

Problem

The Usage → Pricing tab shows an empty state reading "No pricing overrides" with the body copy "Without pricing overrides, costs use the built-in model pricing table. Add custom prices here for specific models." However, the pricing panel is a read-only table with no UI to add, edit, or delete custom pricing overrides. The empty-state copy promises a capability the interface does not deliver.

The backend is fully implemented but the renderer cannot reach it:

• IPC handlers (apps/desktop/src/main/usage-ipc-main.ts): usage:pricing:list, usage:pricing:put, usage:pricing:reset, and a usage:pricing:changed push event are all registered.
• Preload bridge (apps/desktop/src/preload/preload.ts): Does not expose any pricing IPC methods — the renderer cannot call them.
• Pricing panel (apps/desktop/src/renderer/settings/usage-settings-page.tsx): UsagePricingPanel is a read-only table; there is no add/edit/delete form or action button.

Desired outcome

A user can:

1. Add a custom pricing override for a specific model (entering modelKey, inputUsdPer1M, outputUsdPer1M, and optionally cacheReadUsdPer1M / cacheWriteUsdPer1M).
2. Edit an existing pricing override.
3. Delete a pricing override (revert to the built-in pricing table).
4. See the panel refresh automatically after each mutation (the usage:pricing:changed event is already wired on the main-process side).

Alternatives or workarounds

None available today. Users cannot customize model prices through the UI. All costs are settled using the built-in model pricing table, with no override capability despite the backend infrastructure being fully in place.

How to reproduce

1. Open Settings → Usage
2. Click the "Pricing" tab
3. Observe the empty state: "No pricing overrides" / "Without pricing overrides, costs use the built-in model pricing table. Add custom prices here for specific models."
4. Notice there is no "Add" button, form, or any way to create a pricing override.

Environment

• Maka version or commit: 1b0137c1
• OS: All platforms
• Surface: Desktop

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问题

使用统计 → 定价配置 tab 显示空状态"暂无定价覆盖配置"，说明文字为"未配置定价覆盖时，费用按内置模型定价表结算；在此可为特定模型登记自定义价格。"但定价面板只是一个只读表格，没有任何添加、编辑或删除自定义价格的入口。空状态文案承诺了一个界面并未提供的能力。

后端已完整实现，但渲染进程无法调用：

• IPC 处理器（apps/desktop/src/main/usage-ipc-main.ts）：已注册 usage:pricing:list、usage:pricing:put、usage:pricing:reset 以及 usage:pricing:changed 推送事件。
• Preload bridge（apps/desktop/src/preload/preload.ts）：未暴露任何定价 IPC 方法——渲染进程无法调用。
• 定价面板（apps/desktop/src/renderer/settings/usage-settings-page.tsx）：UsagePricingPanel 是只读表格，没有添加/编辑/删除表单或操作按钮。

期望结果

用户可以：

1. 为特定模型添加自定义定价覆盖（输入 modelKey、inputUsdPer1M、outputUsdPer1M，可选 cacheReadUsdPer1M / cacheWriteUsdPer1M）。
2. 编辑已有的定价覆盖。
3. 删除定价覆盖（恢复使用内置定价表）。
4. 每次操作后面板自动刷新（usage:pricing:changed 事件已在主进程侧接好）。

替代方案

目前无任何替代方案。用户无法通过 UI 自定义模型价格，所有费用只能按内置定价表结算。

复现步骤

1. 打开设置 → 使用统计
2. 点击"定价配置"tab
3. 观察空状态："暂无定价覆盖配置" / "未配置定价覆盖时，费用按内置模型定价表结算；在此可为特定模型登记自定义价格。"
4. 注意没有"添加"按钮、表单或任何创建定价覆盖的方式。

环境

• Maka 版本或 commit：1b0137c1
• 操作系统：所有平台
• 界面：Desktop