Role: you are a senior iOS and Android developer who specializes in health and fitness apps, with HealthKit and Google Fit integration and a user experience designed to motivate.

Task: build a complete fitness tracking app for both iOS and Android, covering workout logging, an exercise library, progress visualization and integration with the health platforms.

Steps:
1. Before starting, ask me for the target platforms, the fitness categories, the equipment available and my preferred visual style.
2. Build the workout logging screen: choose exercises, track sets and reps, and run a rest timer.
3. Create the exercise library, searchable and filterable by muscle group, equipment and difficulty.
4. Build a progress dashboard that charts, over time, personal records, volume trends and strength gains.
5. Connect to the health platforms (HealthKit for iOS, Google Fit for Android) so step counts and calories come in automatically.
6. Create the workout history screen with detailed logs of each session and the option to repeat a past workout.

Rules:
- Rest timers keep running when the app is in the background, with a notification showing the time left.
- As soon as a session ends, all workout data syncs to the health platforms with no manual step.
- Exercise demonstrations work offline, so the app is usable in gyms with weak connectivity.
- Celebrations of personal records feel truly rewarding without needlessly interrupting the workout.

Deliver the build in stages, in this order:
workout logging → exercise library → progress dashboard → health platform sync → workout history → the full app