Add per-account folder customization page (#855)

Introduce a dedicated settings page that lets users reorder, hide,
and pin/unpin folders per account. Folders are organized into Pinned,
Categories (Gmail only), and More sections with drag-to-reorder via
ListView. New Order column on MailItemFolder persists the custom
layout; the default sort falls back to alphabetic when no custom
order is set. A reset action wipes all customization in a single
transaction and restores system-folder stickiness.

Co-authored-by: Claude <noreply@anthropic.com>

Wino.Mail.WinUI/Services/NavigationService.cs

@@ -74,6 +74,7 @@ public class NavigationService : NavigationServiceBase, INavigationService
         WinoPage.ManageAccountsPage,
         WinoPage.AccountManagementPage,
         WinoPage.AccountDetailsPage,
+        WinoPage.FolderCustomizationPage,
         WinoPage.MergedAccountDetailsPage,
         WinoPage.SignatureManagementPage,
         WinoPage.AboutPage,
@@ -136,6 +137,7 @@ public class NavigationService : NavigationServiceBase, INavigationService
             WinoPage.None => null,
             WinoPage.IdlePage => typeof(IdlePage),
             WinoPage.AccountDetailsPage => typeof(AccountDetailsPage),
+            WinoPage.FolderCustomizationPage => typeof(FolderCustomizationPage),
             WinoPage.MergedAccountDetailsPage => typeof(MergedAccountDetailsPage),
             WinoPage.AccountManagementPage => typeof(AccountManagementPage),
             WinoPage.ManageAccountsPage => typeof(AccountManagementPage),