Need a habit-tracker app built: daily streaks and reminders

I want to build a mobile app that helps people stick to daily habits, and I'm looking for a developer to take it from idea to a working release on iOS and Android. The core idea is simple: a person adds habits they want to build, things like drinking water, reading, or cutting screen time before bed, and marks each one done with a single tap once a day. The app needs a running streak counter per habit, showing how many days in a row it's been completed, plus a longest-streak record so progress is visible over time.

Beyond the daily check-in, I need a monthly view that visualizes progress, something like a calendar grid or heatmap where completed days stand out, so patterns become obvious: which habits stick, which days tend to get missed. Reminders matter too, each habit should have its own push notification with a chosen time, and ideally a gentle nudge if a streak is about to break.

I'd like the tone to feel calm and encouraging rather than guilt-driven, avoid harsh warning colors or shaming language. Data should sync across devices with a simple account system, stored securely. Open to discussing the stack, but I need someone who has shipped a similar app before, can show finished examples, plus tested push notifications and a documented backend for check-ins and streaks.