Universal Sing-Box VPN App Suite

I already run a sing-box based VPN infrastructure with its own authentication, usage-tracking and subscription system. What I need now is a polished, fully cross-platform client built from one shared codebase—Flutter is my preferred choice, but I’m open to other single-language stacks if they compile cleanly for all targets.

Scope of work
• Build native-feeling apps for Android, iOS, Windows, macOS and Linux. tvOS and OpenWRT builds are nice-to-have extras, not hard requirements.
• Replace the stock sing-box GUI with a modern custom design that matches my brand guidelines.
• Wire the app into my existing backend for:
– User authentication (token-based)
– Real-time data-usage statistics
– Subscription status / renewals
– Remote feature flags so I can toggle experimental protocols without republishing the app
– Remote announcements (simple JSON feed displayed in-app)

Technical notes
• sing-box already handles the transport layer; you’ll wrap its core binaries/libraries and expose the key actions (connect, disconnect, select node, view logs).
• Code must compile from the same repository for every platform, with platform-specific glue kept minimal.
• Clean architecture, strong null-safety and clear separation between UI, data layer and sing-box bindings are essential.
• Deliver CI scripts (GitHub Actions or similar) that generate signed builds for each platform.

Acceptance criteria
1. Successful login, node selection and connection on every required OS.
2. Accurate real-time usage reporting that matches server stats within 1 %.
3. Subscription expiry handled gracefully with an upgrade prompt.
4. Remote flag update reflected in the UI without app restart.
5. Source code, build instructions and a short video demo provided.

If this end-to-end challenge excites you, let’s discuss the sing-box hooks and UI mock-ups so you can get started right away.