refactor: pure-Dart refactor of core packages

[badnikhil]

PR Description

This PR implements a significant architectural refactoring aimed at decoupling Internal logic from the Flutter SDK. This transition is a prerequisite for high-performance, standalone CLI execution.
Key Changes:

• Refactored apidash_core, better_networking, and genai to be pure-Dart packages by removing all hard dependencies on the Flutter framework.
• Introduced isThisWeb and localized platform detection utilities within the pure-Dart packages to replace kIsWeb from the Flutter foundation library.
• Decoupled the OAuth2 flow by introducing a callback-based abstraction. The UI browser logic (flutter_web_auth_2) has been moved to the root application and is now injected into the networking engine at runtime via an OAuth2CallbackHandler.
• Added logic to lib/services/oauth_services.dart to maintain full OAuth2 functionality in the GUI, while allowing the engine for terminal-based auth flows.

Related Issues

• Closes [Bug]: genai package has unnecessary Flutter SDK dependency preventing pure-Dart CLI usage #1591 , bug: better_networking package has Flutter-specific dependencies blocking pure Dart CLI usage #1360

Checklist

• I have gone through the contributing guide
• I have updated my branch and synced it with project main branch before making this PR
• I am using the latest Flutter stable branch (run flutter upgrade and verify)
• I have run the tests (flutter test) and all tests are passing

Added/updated tests?

• Yes
• No, and this is why: Not Required.

OS on which you have developed and tested the feature?

• Windows
• macOS
• Linux