(new) Added offline page

[FussuChalice]

Description

This pull request introduces a new feature: the "Offline Page." In situations where the user's device loses internet connectivity, this page will be displayed, providing a better user experience and useful information.

Changes Made

• Created a new "offline_page.dart" file.
• Designed a user-friendly and informative offline page with appropriate styling.
• Implemented a mechanism to detect and trigger the display of the offline page when the user is offline.
• Included relevant assets and resources to enhance the offline page's appearance and functionality.

How to Test

To verify the changes, follow these steps:

1. Open the app.
2. Disable your internet connection or put your device in Airplane Mode.
3. Observe that the "Offline Page" is displayed.
4. Ensure the page provides helpful information and has a pleasing design.
5. Re-enable your internet connection to see if the offline page disappears when the connection is restored.

Screenshots

[d-loose]

Thanks for your contribution! Sorry, I was busy with the 23.10 release and other things, so I didn't have a change to review the other PRs you opened. I'll have a look at this one later today or tomorrow and will give you some feedback.

[FussuChalice]

Fine. Thank you

[d-loose]

• The page doesn't display properly when using the light theme
  
• We previously used YaruAnimatedNoNetworkIcon provided by yaru_widgets, which is a better choice than a raster image asset.
• This solution only handles the case where there's no internet connection when the application is started - it'd be great if we could react to connectivity changes without restarting the application. We used connectivity_plus before, but there's still an unresolved upstream issue, previously discussed in Store is offline on a netplan managed connection  #674. Also see First run without internet connection #1432 (comment)

It's probably useful to get some feedback from @anasereijo as well :)