feat: Refactor code and separate portal API calls

- Moved portal API calls to a separate class for better code organization.
- Updated `AttendanceWorker`, `AttendanceService`, and `MainViewModel` to use the new `portal` class.
- Removed unused imports in several files.
- Renamed `AttendanceView` and `Screen` classes to match the new file structure.

feat: refactor file structure and rename files

- Rename Auth.kt to auth.kt
- Move SettingsView.kt to views package
- Adjust imports due to file relocation in MainActivity.kt

feat: extract Reglab related functionalities to separate class

This commit includes the extraction of Reglab related functionalities from the MainViewModel into a separate Reglab class. This separation of concerns improves readability and maintainability. Changes were also made to the ReglabView to use the new Reglab class for fetching schedule data.

feat: add fetch schedule data function in MainViewModel

This commit adds a new function `fetchScheduleData` in the file `MainViewModel.kt`. This function retrieves the schedule data for a user's practicum. A Gson library is also imported to parse the JSON response.

feat: disable minify for release build type in build.gradle.kts

feat: add more classes to proguard-rules configuration

This commit adds several classes to the proguard-rules.pro configuration in the app. These additions prevent warnings from these classes when building the application. The classes are mainly from Google's API client and errorprone annotations.

feat: Refactor codebase and organize imports

This commit includes reorganizing the codebase by relocating certain classes to their appropriate packages. It also involves adjusting imports accordingly across various files. A few changes to some classes are included as well.

feat: add network availability check

- Added check for network availability before initializing attendance worker
- Included network availability check before marking attendance
- Displayed toast message when network is not available in MainActivity
- Added network availability check in MainViewModel
- Added retry mechanism in Auth.kt for handling network timeouts and disconnections

feat: add tester recruitment section in README.md

feat(README.md): update images and layout