feat: add native HarmonyOS / 鸿蒙 app support

## Feature request

Please consider adding an official **HarmonyOS / 鸿蒙** client for Cindy.

Cindy already targets desktop and mobile platforms, and an official HarmonyOS app would make the agent much more accessible to users in the growing Huawei / HarmonyOS ecosystem, especially on HarmonyOS NEXT devices where Android compatibility is no longer the long-term solution.

## Why this would be useful

- **Native HarmonyOS NEXT support** instead of depending on Android compatibility.
- Bring Cindy's mobile AI-agent experience to Huawei phones and tablets.
- Improve coverage for users in China, where HarmonyOS devices have a significant user base.
- Cindy's open-source architecture makes it a good candidate for community participation around a new platform implementation.

## Suggested MVP

A first version does not need full platform parity. It could focus on:

1. Account login and session synchronization.
2. Chat / agent task creation and task history.
3. Remote task status and result viewing.
4. File/image attachment upload.
5. Push notifications for completed tasks or requests requiring user confirmation.
6. Basic settings and model/account configuration supported by the existing backend.

Later versions could integrate HarmonyOS-native capabilities such as notifications, intents/skills, file picker, sharing, microphone/voice input, background tasks, and cross-device collaboration.

## Implementation consideration

If reusing the existing React Native mobile layer is not practical for HarmonyOS NEXT, an **ArkTS + ArkUI** client that shares API/protocol definitions and business models with the existing clients may be a reasonable path.

It would also be helpful if the maintainers could clarify whether HarmonyOS support is already on the roadmap, and whether community contributions for an initial HarmonyOS client would be welcome.

Thanks for considering it.

Start by reviewing Cindy’s existing React Native mobile layer, then identify the shared API/protocol definitions and business models that could support a HarmonyOS client. Compare reuse of that layer with an ArkTS + ArkUI implementation. Done means an initial client covering login and session sync, chat and task history, remote status and results, attachments, notifications, and basic settings.

Written by the indexing model from the issue text.