Migrate to Provider or Riverpod State Management

[MasterAffan]

Description:
The current Flutter app uses basic setState and manual state handling. Migrate the app to use Provider or Riverpod for cleaner, scalable, and maintainable state management.

Tasks:

• Decide between provider or flutter_riverpod for the project (align with team preference).
• Add the chosen package to pubspec.yaml.
• Refactor major screens and widgets to use the new state management pattern.
• Move app-wide state (user data, workout progress, theme, etc.) into Providers.
• Remove redundant setState calls and unnecessary StatefulWidget usage.
• Ensure navigation and async data flows (API calls, theme changes, AI responses) still function correctly.
• Test and verify app behavior matches the previous version.

Acceptance Criteria:

• App runs with Provider/Riverpod managing all reactive data.
• No reliance on manual setState for global or shared data.
• State updates are efficient and predictable.
• All major app features (login, chat, video analysis, theme) work as before.
• Code structure is modular and easy to maintain.