DSH: deliver the capability-gated Desktop core task flow

## 目标

让 Desktop 用户创建、继续、观察、批准、取消和恢复同一 DSH task，并如实显示 runtime/Cindy capability 状态。

## 交付

- selector/availability、create/resume、model/effort/attachment gate、timeline、approval/cancel/reconcile UI。
- named schema-validated Main/preload actions；不暴露 Home、ACP transport、secret 或 arbitrary command。
- 五 locale 与 Light/Dark 交付。

## 验收

- 本地真实 DSH task 可创建到恢复；未支持 action 显示真实原因。
- DSH 不会被替换成其它 Agent。

Blocked by #3775. Blocks #3777 and #3778.

Canonical plan: `docs/issues/dsh-native-integration/`.

Start with the canonical plan in docs/issues/dsh-native-integration/ and resolve the dependency on #3775 before implementation. Review the Desktop Main/preload action requirements and acceptance criteria; the work is done when a real DSH task can be created through recovery, unsupported actions report their actual reason, and the stated locale and theme coverage is delivered.

Written by the indexing model from the issue text.