feat: 预计最终到期时间展示

openspec/changes/add-asset-estimated-final-expiry-display/specs/device-management/spec.md

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+## ADDED Requirements
+
+### Requirement: Device Estimated Final Expiry Display
+
+The device management list integration SHALL preserve and display the backend device-level estimated final package expiry fields without traversing bound cards or calculating package continuation dates in the frontend.
+
+#### Scenario: Display exact device final expiry estimate
+
+- **GIVEN** 设备列表接口返回 `expiry_estimate_status=exact` and `estimated_final_expires_at`
+- **WHEN** 页面渲染设备列表行
+- **THEN** 页面 MUST display a column labeled `预计套餐到期时间`
+- **AND** 该列 MUST format and display `estimated_final_expires_at`
+
+#### Scenario: Display activation-pending device final expiry
+
+- **GIVEN** 设备列表接口返回待激活等不可预计的 `expiry_estimate_status`
+- **WHEN** 页面渲染设备列表行
+- **THEN** `预计套餐到期时间` MUST display `待激活后起算`
+- **AND** 页面 MUST NOT traverse bound cards or display a fabricated date
+
+#### Scenario: Highlight expiring device without reordering
+
+- **GIVEN** 设备列表接口返回 `is_expiring=true` and `days_until_final_expiry`
+- **WHEN** 页面渲染该设备的预计套餐到期时间
+- **THEN** 页面 MUST apply the expiring visual treatment based on the backend fields
+- **AND** 页面 MUST NOT change the ordinary device list sort order because of `is_expiring`
+
+#### Scenario: Display no-package device placeholder
+
+- **GIVEN** 设备列表记录没有可预计的最终到期时间
+- **WHEN** 页面渲染预计套餐到期时间列
+- **THEN** 页面 MUST display a stable placeholder
+- **AND** 页面 MUST NOT use an individual package expiry as a substitute