Improve Avalonia UI and fix Linux build issues

[Gabriel Dufresne (GabrielDuf)]

Summary

Align the Avalonia UI more closely with the WinUI version across colors, controls, and layout. it also fix linux build issues

Theme & color palette

• Neutral grays everywhere. Replaced the blue-tinted dark theme (#191b29, #222534, …) with WinUI's neutral
  Mica-style grays (#202020, #2D2D2D, #F3F3F3, #FBFBFB, …).
• Fluent v2 banner colors. Status/warning banners now use the WinUI SystemFillColor* palette (#FCE100,
  #6CCB5F, #FF99A4, etc. in dark; #9D5D00, #0F7B0F, #C42B1C, etc. in light) instead of the prior
  Material-flavored hues.
• WinUI Fluent v2 token polyfill. Avalonia 12's Fluent theme ships only UWP-era resource names. Added the missing
  v2 set (AccentFillColor*, AccentTextFillColor*, TextFillColor*, TextOnAccentFillColor*) with proper
  Light/Dark variants in Styles.Common.axaml, so accent text matches the checkbox accent and other Fluent-styled
  controls.

Filter pane — Sources tree

• Added expand/collapse chevron for nodes with children (winget's sub-sources). Chevron sits between the checkbox box
  and the package name with Opacity="0.55", only visible when HasChildren is true.
• SourceTreeNode.HasChildren raises PropertyChanged on Children mutations so the chevron appears reactively as
  sources load.
• "Select all" / "Clear selection" use AccentTextFillColorPrimaryBrush (the new v2 token) for accent text.

Package list

• DataGrid selection: removed full-row accent-blue fill. Selected rows show only a 2px accent border via constant
  BorderThickness with BorderBrush=Transparent for unselected rows — so toggling selection no longer shifts
  adjacent rows.
• GridLines off: hides the horizontal divider between rows that previously persisted under the new selection
  style.
• Tri-state "Select all" checkbox: ObservablePackageCollection now raises SelectionStateChanged whenever a
  PackageWrapper.IsChecked toggles or items are added/removed. The ViewModel recomputes AllPackagesChecked to
  true/false/null (none/some/all), and IsThreeState="True" on both Select-all CheckBoxes renders the
  indeterminate dash.