feat(app): first-run data location wizard step

On first launch (no data yet, no portable dir, no pointer file),
the wizard now shows a new step-0 screen asking where to store
wallets, config and certificates.

AppPaths changes:
- DefaultDataRoot() → ~/.PalladiumWallet (home, always writable)
- IsDataLocationConfigured() → true when portable / override / pointer
  already written / legacy or default already has data
- ConfigureDataLocation(root) writes a bootstrap pointer file and
  creates the directory
- DataRoot() resolution order: override → portable → pointer → legacy
  (has data) → default

ViewModel: new StepDataLocation step, UseDefaultDataLocationCommand,
ApplyDataLocation(root) (also called from View's folder picker).

View: new wizard panel with description, default-path display, two
buttons (use default / choose folder); folder picker via
StorageProvider.OpenFolderPickerAsync.

Loc: add wiz.data.* keys (6 languages); fix fallback language "it"→"en";
update test assertion accordingly.

src/App/Views/MainWindow.axaml.cs

@@ -72,6 +72,21 @@ public partial class MainWindow : Window
             vm.IsServerSettingsOpen = false;
     }
 
+    private async void OnChooseDataFolderClick(object? sender, RoutedEventArgs e)
+    {
+        if (DataContext is not MainWindowViewModel vm)
+            return;
+
+        var folders = await StorageProvider.OpenFolderPickerAsync(new FolderPickerOpenOptions
+        {
+            Title = "Cartella dati Palladium Wallet",
+            AllowMultiple = false,
+        });
+
+        if (folders.FirstOrDefault()?.TryGetLocalPath() is { } path)
+            vm.ApplyDataLocation(path);
+    }
+
     private void OnConnectionStatusTapped(object? sender, TappedEventArgs e)
     {
         if (DataContext is MainWindowViewModel vm)