Refine avalonia UI

[Gabriel Dufresne (GabrielDuf)]

Refine the Avalonia UI: native Windows 11 styling, animations, and polish Brings the Avalonia build of UniGetUI closer to the look and feel of the original WinUI app, plus a number of UI fixes and platform-specific touch-ups. Most of the work is Windows-only and cleanly isolated so macOS and Linux are unaffected.

Windows 11 Mica + native styling

• Mica backdrop on the main window (whole-window), with native rounded corners. Restores the MicaBackdrop look the WinUI build had.
• Bumped Avalonia 12.0.3 → 12.0.4, which carries the framework fix (PR #21354 / issue #21082) for transparency rendering black on Windows 11.
• Secondary windows / dialogs (Package Details, Install Options, About, etc.) get the same Mica backdrop + an accent colored border that follows focus, via a reusable MicaWindowHelper.
• Translucent “card” surfaces (settings cards, package grid/icons cards, panels, badges, menus, expanders, tabs) so they inherit the Mica tint instead of reading as flat grey.
• Strict platform isolation: the original opaque values stay in Styles.Common.axaml; all translucent/Mica overrides live in a new Styles.WindowsMica.axaml that is merged only when Mica is actually usable (Windows 11 + the OS “Transparency effects” setting). macOS, Linux, Windows 10, and transparency-off all keep the solid look and never flash black.
• Flyout/menu/dropdown/tooltip popups get the app’s Mica backdrop (via a PopupRoot hook) with the card tint, so menus match the rest of the app.

Animations (WinUI parity)

• Page switches use a fade + slide-up entrance, dropping the outgoing page so the two never overlap.
• Settings sub-pages slide horizontally in a direction that follows navigation (drill-in from the right, back from the left), with scrollbars hidden during the slide and no blank/flicker frame.
• Filter pane casts a BoxShadow matching the WinUI ThemeShadow elevation (most visible in overlay mode).

Tray & macOS

• New monochrome tray icon style and unified tray-icon selection (with the existing legacy-icon option).
• macOS app icon reworked to a declarative Icon Composer asset with a committed .icns, fixing the release build / Dock appearance.