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/AppPaths.cs

@@ -18,9 +18,12 @@ public static class AppPaths
         if (Directory.Exists(portable))
             return portable;
 
-        return OperatingSystem.IsWindows()
-            ? Path.Combine(Environment.GetFolderPath(Environment.SpecialFolder.ApplicationData), "PalladiumWallet")
-            : Path.Combine(Environment.GetFolderPath(Environment.SpecialFolder.UserProfile), ".palladium-wallet");
+        // Windows → %APPDATA%\PalladiumWallet
+        // Linux   → $XDG_CONFIG_HOME/PalladiumWallet  (default ~/.config/PalladiumWallet)
+        // macOS   → ~/Library/Application Support/PalladiumWallet
+        return Path.Combine(
+            Environment.GetFolderPath(Environment.SpecialFolder.ApplicationData),
+            "PalladiumWallet");
     }
 
     /// <summary>Cartella dati della rete (config, wallet, header, certificati).</summary>