feat: Add comprehensive "About" dialog

This commit introduces a new "About" dialog that provides users with comprehensive information about the application. An "Info" icon has been added to the main screen's top app bar to launch this dialog.

-   **UI & Dialog Implementation:**
    -   Adds `AboutDialog.kt`, a new composable for displaying detailed application information in a dedicated `DialogWindow`.
    -   The dialog is accessible via a new `Info` `IconButton` in the `TopAppBar` on the main `RepoScreen`.

-   **Content & Information:**
    -   **App Details:** Displays the application icon, name (`Git Backup Hub`), and version (`1.0.0`).
    -   **Key Features:** Lists the application's main functionalities, such as fetching repositories, handling private repos, selective backups, and progress tracking.
    -   **How-To Guides:** Provides step-by-step instructions on how to use the application and how to generate a GitHub Personal Access Token (PAT), including a direct link to the GitHub token settings page.
    -   **Tech Stack:** Details the technologies used, including Compose Multiplatform, Ktor, and Koin.
    -   **Project Links:** Includes buttons to view the project on GitHub, report an issue, and a call-to-action for feature requests.
    -   **Attribution & Support:** Features a "Created by" section and encourages users to star the project on GitHub.

Author: Coding-Meet