Basic Cross-Platform Classifieds Prototype

I need a stripped-down MVP of a classifieds application that runs on both iOS and Android. The goal is to prove the concept at the lowest possible cost, so I am happy with a lean, utilitarian interface built with any efficient cross-platform stack such as Flutter or React Native—whatever gets us to a working prototype fastest.

Core scope
• User registration and login: social sign-in is optional, email + password is enough.
• Posting classified ads: title, short description, price, one photo placeholder is fine for now.
• Search functionality: keyword search across titles and descriptions, very basic filtering (category or price range) if it adds minimal effort.
• In-app messaging: when someone taps “Contact seller,” a simple chat thread opens; no need for push notifications in this phase, just in-app threads stored locally or in a lightweight backend.

Acceptance criteria
• A single codebase successfully compiled and installed on both an iOS test device and an Android test device.
• Clean, intuitive navigation that allows a new user to register, post an ad, find it through search, and initiate an in-app message with the seller without crashing.
• Clear hand-off: source code, brief README on how to build/run, and login credentials for a demo account.

If an off-the-shelf backend (Firebase, Supabase, etc.) accelerates delivery, please integrate it; otherwise a simple local JSON store is acceptable. The design can mirror default system components—no custom UI needed beyond basic branding colours I will supply later.

Timeline is flexible within reason, but I would like to see a clickable build within a week so I can start user testing.