Avalonia: add splash screen + fix package card / search bar visual issues

[Gabriel Dufresne (GabrielDuf)]

Summary

Avalonia-side polish covering three independent visual fixes plus a new splash screen.

• Splash screen — Avalonia had no splash at startup (WinUI gets one via the MSIX uap:SplashScreen manifest entry, which Avalonia can't piggyback on). New SplashWindow
  shows a rounded themed card with the UniGetUI branding while package managers and the main window load. ApplyTheme(...) now runs before the splash is constructed so it starts in
  the user's preferred light/dark variant, and the splash is closed on the first MainWindow.Opened. Daemon-mode launches skip it.
• Themed splash artwork — replaced SplashScreen.png with the new light-theme branded image and added SplashScreen.theme-dark.png. Windows picks the dark variant
  automatically via the theme-dark MRT qualifier; Avalonia swaps the source in SplashWindow.axaml.cs based on Application.Current.ActualThemeVariant.
• Grid-view card alignment — in the packages grid view, the "···" overflow button was visibly off-center vs. the checkbox above it (left-aligned TextBlock inside the 22×22
  button + default CheckBox padding). Fixed by centering the TextBlock and zeroing CheckBox padding; same TextBlock fix applied to the icons-view card.
• Mega-query search bar — the search field and search button no longer have mismatched borders. Wrapped both in a single outer Border with ClipToBounds so they share one
  continuous rounded edge.
• Title-bar search bar — same unified-border treatment; switched the background from AppWindowBackground (identical to the title bar) to AppBadgeBackground for visible
  contrast, and removed the leftover Opacity="0.6" that was washing the field out in light mode.