Reorganize repository directory structure and general clean up

[hjiangsu]

Pull Request Description

This PR cleans up and reorganizes the repository to make it easier to navigate and maintain! I've reorganized it such that it now loosely follows a feature-first, clean architecture approach.

Here's a high-level summary of the changes that were made:

• The lib directory now only contains l10n and src. All localization files are kept in l10n and l10n/generated.
• The src directory has been reorganized into app, core, features, and shared directories:
  • app: contains the main entry point for Thunder.
  • core: contains core logic such as the database.
  • shared: contains shared widgets and functions that are used across features.
  • features: contains domain-specific widgets and logic.

Aside from the reorganization, there was also some cleanup done:

• Cleaned up the settings pages and reduced unnecessary usages of SliverToBoxAdapter
• Removed macos, linux, and windows directories since these platforms are not officially supported. They can be re-added anytime using: flutter create --platforms=windows,macos,linux .
  • Note: I've decided to keep the web platform even though its not officially supported as there was previous work done on it.

Issue Being Fixed

Issue Number: N/A

Screenshots / Recordings

Checklist

• If a new package was added, did you ensure it uses an appropriate license and is actively maintained?
• Did you use localized strings (and added appropriate descriptions) where applicable?
• Did you add semanticLabels where applicable for accessibility?