Correction of flutter errors + Native iOS/Android

Due to overpromising and developers exaggerating their skills I am sorry for posting this again.

Wednesday until today Friday: Yet another developer who now says he has electricity power cuts, too many add ins (and the app is in production already). Can anyone bring some competent skilled developers to me please who wants to provide evidence of skills and not say "I can do it" as proof?

And no, it is not anything wrong with my requirements, but when people say they have developed ios apps and then say I should approve udid then something is not right.
So please do your self a favour, do not exaggerate as it is easy to spot.

in this case you must repair errors are repaired by printing on screens and log what is happening. This is like having a windshield on your car and see what you are doing. Sitting around and guessing does not work here, neither accusing or blaming either the code, other developers or me.
This is house-training for developers with professional skills.

Now, down to business:

Status
- App is live on both stores and stable.
- Codebase upgraded to Flutter 3.32.x with updated dependencies.
- Builds run, but previous developer failed to resolve remaining issues.

Issues to Fix

Core Functional Issues
1. iOS (Xcode 16, iOS SDK 17/18) — Selfie screen responsiveness: requires ~20 taps before reacting.

2–3. Method Channel / API Call Layer (related issues)
- Android (SDK 34) — Incorrect color payloads to server: stores black (= no color) instead of selected value.
- iOS Rendering — Green tint applied to faces (native layers OK; problem is in Flutter bridge).

4. iOS Slider Performance — Opacity slider stutters/lags; updates on every drag tick instead of on release (Android is smooth).

Simple UI Fixes
5. Navigation — Back arrow in selfie/analysis flow not working.
6. UI Cleanup — Remove line around button.
7. iOS Product Visualisation — Colours appear low/washed-out in product screens.
8. UI Scaling — Oversized buttons in some views.

Requirements (Must-Have)
- Senior Flutter engineer with strong Swift + Kotlin background.
- macOS with Xcode 16 + iOS SDK 17 device/simulator.
- Hands-on fixing: Flutter.h, modulemap, use_frameworks!, Bitcode off, xcframework conversions, plugin incompatibilities.
- Must produce signed release builds and upload to TestFlight v2.0.

Deliverables
- Fixed: selfie responsiveness, method channel issues (color persistence + green tint), slider smoothness, navigation, UI fixes.
- Release builds:
- iOS → uploaded to TestFlight v2.0
- Android → signed, ready for store
- Strictly bug fixes & compatibility. No new features.

Targets
- iOS: Xcode 16, iOS SDK 17/18, min iOS 13, Bitcode disabled, xcframework where required.
- Android: Target SDK 34, updated Gradle & Kotlin, proper signing for release.

Timeline
- Start: Immediately
- Delivery: 2–3 days maximum

Work Ethic & Professionalism
No juniors. No guesswork. No deflection.
I have 48 years in software, I expect precision and professionalism.
If you solve problems without excuses, apply.

Additional Requirement
- Push the app to both stores:
- TestFlight for iOS
- Google Play Test Track (APK) for Android

PS! Your role is also to validate the code is updated correctly.