feat(app): settings menu with language and display unit

Add global AppConfig (config.json, blueprint §8) persisting language
and amount unit. CoinAmount gains unit-aware formatting and parsing
(PLM, mPLM, µPLM, sat), applied everywhere amounts are shown or
entered, including the send form. New Loc i18n layer (it/en) with
live-updating XAML bindings covering menu, wizard, wallet panel and
status messages.

The Impostazioni menu opens a dropdown with separate Lingua and Unità
entries (radio-checked submenus, applied and saved on click), ready to
host future settings.

src/Core/Storage/AppConfig.cs

@@ -0,0 +1,42 @@
+using System.Text.Json;
+
+namespace PalladiumWallet.Core.Storage;
+
+/// <summary>
+/// Configurazione globale dell'applicazione (blueprint §8), separata dai file
+/// wallet: lingua, unità di visualizzazione, ecc. Persistita in config.json
+/// nella radice dei dati (vale per tutte le reti).
+/// </summary>
+public sealed class AppConfig
+{
+    /// <summary>Codice lingua UI ("it", "en").</summary>
+    public string Language { get; set; } = "it";
+
+    /// <summary>Unità di visualizzazione degli importi (vedi <see cref="Wallet.CoinAmount.Units"/>).</summary>
+    public string Unit { get; set; } = "PLM";
+
+    private static readonly JsonSerializerOptions JsonOptions = new() { WriteIndented = true };
+
+    public static AppConfig Load(string? path = null)
+    {
+        path ??= AppPaths.ConfigPath();
+        if (!File.Exists(path))
+            return new AppConfig();
+        try
+        {
+            return JsonSerializer.Deserialize<AppConfig>(File.ReadAllText(path)) ?? new AppConfig();
+        }
+        catch (JsonException)
+        {
+            // Config corrotta: si riparte dai default senza bloccare l'avvio.
+            return new AppConfig();
+        }
+    }
+
+    public void Save(string? path = null)
+    {
+        path ??= AppPaths.ConfigPath();
+        Directory.CreateDirectory(Path.GetDirectoryName(path)!);
+        File.WriteAllText(path, JsonSerializer.Serialize(this, JsonOptions));
+    }
+}