feat(storage): XDG-compliant paths and default language English

AppPaths: use Environment.SpecialFolder.ApplicationData for all platforms
  - Windows  → %APPDATA%\PalladiumWallet  (unchanged)
  - Linux    → ~/.config/PalladiumWallet  (XDG standard, was ~/.palladium-wallet)
  - macOS    → ~/Library/Application Support/PalladiumWallet

AppConfig: change default language from "it" to "en" for new installs.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/Core/Storage/AppConfig.cs

@@ -9,8 +9,8 @@ namespace PalladiumWallet.Core.Storage;
 /// </summary>
 public sealed class AppConfig
 {
-    /// <summary>Codice lingua UI ("it", "en").</summary>
-    public string Language { get; set; } = "it";
+    /// <summary>Codice lingua UI.</summary>
+    public string Language { get; set; } = "en";
 
     /// <summary>Unità di visualizzazione degli importi (vedi <see cref="Wallet.CoinAmount.Units"/>).</summary>
     public string Unit { get; set; } = "PLM";