feat: 换货退款拦截错误展示

openspec/changes/update-exchange-refund-interception-feedback/specs/exchange-management/spec.md

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+## ADDED Requirements
+
+### Requirement: Exchange Creation Refund Interception Feedback
+
+The exchange creation page SHALL display the backend business reason when `POST /api/admin/exchanges` rejects the selected old asset because of an active refund application. The page MUST use the response `code` and `msg` contract without changing the successful exchange response structure.
+
+#### Scenario: Display active refund interception reason
+
+- **GIVEN** 用户已填写创建换货单表单并选择存在活跃退款申请的旧资产
+- **WHEN** `POST /api/admin/exchanges` 返回失败 `code` 和 `msg`
+- **THEN** 页面 MUST display the backend business reason in the create exchange form context
+- **AND** 当后端 `msg` 为“该资产存在退款申请”时，页面 MUST display that message directly
+- **AND** 系统 MUST NOT create a new exchange order
+
+#### Scenario: Display other exchange creation business reason
+
+- **GIVEN** 用户已填写创建换货单表单
+- **WHEN** 创建接口返回非退款相关的业务失败 `code` 和 `msg`
+- **THEN** 页面 MUST display the backend `msg` in the create exchange form context
+- **AND** 页面 MUST NOT replace the backend business reason with a generic frontend failure message
+
+### Requirement: Exchange Creation Retry After Interception
+
+The exchange creation page SHALL preserve the current form values after a backend rejection and allow the user to change assets and submit again after the request completes.
+
+#### Scenario: Preserve form after refund interception
+
+- **GIVEN** 创建换货单请求被后端退款规则拒绝
+- **WHEN** 页面展示后端业务原因
+- **THEN** 页面 MUST retain the selected old asset, new asset when applicable, flow type, reason, and other form values
+- **AND** 提交按钮 MUST leave its loading state after the request completes
+
+#### Scenario: Retry with another asset
+
+- **GIVEN** 用户已收到退款拦截错误
+- **WHEN** 用户更换旧资产并再次提交创建换货单
+- **THEN** 系统 MUST send a new `POST /api/admin/exchanges` request using the updated form values
+
+### Requirement: Backend-Controlled Exchange Refund Eligibility
+
+The exchange creation page MUST rely exclusively on the exchange creation response to determine refund-related eligibility. The frontend MUST NOT add a refund status lookup, a refund pre-check area, or infer eligibility from refund approval, rejection, withdrawal, or completion states.
+
+#### Scenario: Do not pre-check refund eligibility
+
+- **WHEN** 用户打开或填写创建换货单表单
+- **THEN** 页面 MUST NOT issue a refund status query solely to determine exchange eligibility
+- **AND** 页面 MUST NOT render a refund status pre-check area
+
+#### Scenario: Permit assets according to backend decision
+
+- **GIVEN** 用户选择曾有退款申请但后端允许换货的资产
+- **WHEN** 创建接口成功返回
+- **THEN** 页面 MUST treat the exchange creation as successful
+- **AND** 页面 MUST NOT block the operation based on a frontend interpretation of refund history