Extract AppCatalogView UserControl from MainWindow

[mock1ngbb]

UI MIGRATION — move ~770 lines of app grid/list view into a dedicated UserControl.

Extract from MainWindow.axaml.cs:

• App catalog display (grid view, list view, card rendering)
• Sorting/filtering logic (SortBy enum handling)
• Game card rendering

New files: Controls/AppCatalogView.axaml + Controls/AppCatalogView.axaml.cs

• Wire via <local:AppCatalogView /> tag in MainWindow.axaml

Verification: dotnet build passes, app catalog displays with all sort/filter options working.
Doable by: sonnet/cline-agent under 45 min.