Turn an Idea into a Product Spec

Background: I want to build a [mobile app type] for [target users].
Platform intent: [iOS / Android / both]. I am a [solo dev / small team]
working with [native SwiftUI+Combine / Jetpack Compose / both].

Use case: Help me turn this rough idea into a structured product spec
I can actually build from.

The idea in my words:
[Describe the app, the problem it solves, and who it's for]

Implementation details: Keep it realistic for a first version. Flag
anything that sounds simple but is actually hard to build on mobile
(real-time sync, background processing, payments, offline support).

Deliverable: A product spec with:
1. One-sentence product definition and target user
2. Core problem and the "job" the user hires the app to do
3. Primary user flows (numbered, step by step)
4. Feature list grouped as "core" vs "nice to have"
5. Platform-specific considerations (iOS vs Android) I should know now
6. Open questions I need to answer before building