Update splash screen svg and translation

[Gabriel Dufresne (GabrielDuf)]

This pull request updates the splash screen in both Avalonia and WinUI versions of the app.

Splash screen and tagline layout improvements:

• Refactored the splash screen layout in SplashWindow.axaml to use a Grid with precise column and row ratios, ensuring the tagline sits directly below the wordmark, and added ClipToBounds="True" for better rendering.
• In the WinUI MainWindow.xaml, replaced the previous stack panel and border structure with a Grid that matches the splash image's aspect ratio, and positioned the tagline using a TranslatedTextBlock for more accurate alignment under the wordmark.

Tagline theming and localization:

• In SplashWindow.axaml.cs, set the tagline text to a translatable string ("Package management made easy") and dynamically adjusted its foreground color based on the current theme (light or dark).

Code and resource updates:

• Added necessary using directives for Avalonia.Media and UniGetUI.Core.Tools in SplashWindow.axaml.cs to support theming and translation features.