Align Avalonia UI with WinUI styling and behavior

[Gabriel Dufresne (GabrielDuf)]

This pull request introduces several improvements to the application's UI consistency and theming, especially for package lists, navigation, and badges, aligning them more closely with WinUI design patterns. It also refactors how package tag icons are handled in the code, and removes an unused theme package reference.

The most important changes are:

UI Theming and Styles

• Added new brushes and styles in Styles.Common.axaml and Styles.WindowsMica.axaml for package lists, column headers, and subtle fills, making backgrounds and hover/press states more visually consistent with WinUI. This includes new resources like PackageListBackground, PackageListHeaderBackground, and subtle fill brushes for both light and dark themes.
• Introduced custom styles for sidebar navigation items (ListBoxItem.nav-item) and segmented view selectors (ListBoxItem.segment-item) to use subtle neutral fills and accent selection indicators, instead of full accent fills.
• Set DataGridColumnHeader font size to 14px to match WinUI, improving column header appearance.

Package Tag Icon Refactor

• Changed package tag icon handling in PackageCollections.cs to use separate properties for the symbol and backdrop SVGs (TagSymbolIconPath and TagBackdropIconPath), allowing for a WinUI-style "punched out" badge effect. Updated logic for which icons are shown and when.
• Updated property change notifications to reflect the new icon property names.

ViewModel Improvements

• Improved labels and tab headers in InstallOptionsViewModel to better match WinUI's two-line tab style and added new translated labels for placeholders and actions.
• Added missing using directives for consistency.

Dependency Cleanup

• Removed the unused Devolutions.AvaloniaTheme.DevExpress package reference from the project file.