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));
+    }
+}

src/Core/Wallet/CoinAmount.cs

@@ -10,10 +10,51 @@ public static class CoinAmount
 {
     public const long SatsPerCoin = 100_000_000;
 
+    /// <summary>Unità di visualizzazione selezionabili (config §8).</summary>
+    public static readonly string[] Units = ["PLM", "mPLM", "µPLM", "sat"];
+
+    /// <summary>(satoshi per unità, decimali mostrati) di ciascuna unità.</summary>
+    private static (long Factor, int Decimals) Of(string unit) => unit switch
+    {
+        "mPLM" => (100_000, 5),
+        "µPLM" => (100, 2),
+        "sat" => (1, 0),
+        _ => (SatsPerCoin, 8), // PLM
+    };
+
     public static string Format(long sats, string unit = "") =>
         (sats / (decimal)SatsPerCoin).ToString("0.00000000", CultureInfo.InvariantCulture)
         + (unit.Length > 0 ? " " + unit : "");
 
+    /// <summary>Formatta nell'unità scelta (es. 150000 sat → "1.50000 mPLM").</summary>
+    public static string FormatIn(long sats, string unit, bool withLabel = true)
+    {
+        var (factor, decimals) = Of(unit);
+        var value = (sats / (decimal)factor).ToString(
+            decimals == 0 ? "0" : "0." + new string('0', decimals), CultureInfo.InvariantCulture);
+        return withLabel ? $"{value} {unit}" : value;
+    }
+
+    /// <summary>Parsa un importo espresso nell'unità scelta in satoshi.</summary>
+    public static bool TryParseIn(string text, string unit, out long sats)
+    {
+        sats = 0;
+        var (factor, _) = Of(unit);
+        text = text.Trim().Replace(',', '.');
+        if (!decimal.TryParse(text, NumberStyles.Number, CultureInfo.InvariantCulture, out var value)
+            || value < 0)
+            return false;
+        try
+        {
+            sats = (long)(value * factor);
+        }
+        catch (OverflowException)
+        {
+            return false;
+        }
+        return true;
+    }
+
     /// <summary>Parsa un importo in coin (punto o virgola decimale) in satoshi.</summary>
     public static bool TryParseCoins(string text, out long sats)
     {