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

@@ -41,6 +41,23 @@
                 <TextBlock Text="Palladium Wallet" FontSize="28" FontWeight="Bold"
                            HorizontalAlignment="Center"/>
 
+                <!-- Passo 0 (primo avvio): dove salvare i dati -->
+                <StackPanel IsVisible="{Binding IsStepDataLocation}" Spacing="12">
+                    <TextBlock Text="{Binding Loc[wiz.data.title]}" FontSize="18" FontWeight="Bold"/>
+                    <TextBlock Text="{Binding Loc[wiz.data.info]}" TextWrapping="Wrap" Foreground="Gray"/>
+                    <StackPanel Spacing="4">
+                        <TextBlock Text="{Binding Loc[wiz.data.default]}" FontSize="11" Foreground="Gray"/>
+                        <SelectableTextBlock Text="{Binding DefaultDataPath}"
+                                             FontFamily="monospace" FontSize="13" TextWrapping="Wrap"/>
+                    </StackPanel>
+                    <Button Content="{Binding Loc[wiz.data.usedefault]}" FontSize="16" Classes="accent"
+                            HorizontalAlignment="Stretch" HorizontalContentAlignment="Center"
+                            Command="{Binding UseDefaultDataLocationCommand}"/>
+                    <Button Content="{Binding Loc[wiz.data.choose]}" FontSize="16"
+                            HorizontalAlignment="Stretch" HorizontalContentAlignment="Center"
+                            Click="OnChooseDataFolderClick"/>
+                </StackPanel>
+
                 <!-- Passo 1: scelta iniziale -->
                 <StackPanel IsVisible="{Binding IsStepStart}" Spacing="12">
                     <StackPanel Orientation="Horizontal" Spacing="10" HorizontalAlignment="Center">