feat(ui): add bug report button in Help and Gitea issue template

Adds a "Report a bug" / "Segnala un bug" button (6 languages) in
Help → Info that opens the repository issue tracker in the system
browser via TopLevel.Launcher (cross-platform: desktop + Android).

Adds .gitea/ISSUE_TEMPLATE/bug_report.md — Gitea-compatible markdown
template with pre-filled sections for version, platform, network,
repro steps, expected behavior, and logs.

src/App/Views/MainView.axaml.cs

@@ -1,3 +1,4 @@
+using System;
 using System.Linq;
 using Avalonia;
 using Avalonia.Controls;
@@ -128,6 +129,14 @@ public partial class MainView : UserControl
             vm.IsHelpOpen = false;
     }
 
+    private async void OnOpenBugReportClick(object? sender, RoutedEventArgs e)
+    {
+        var launcher = TopLevel.GetTopLevel(this)?.Launcher;
+        if (launcher is not null)
+            await launcher.LaunchUriAsync(new Uri(
+                "https://santantonio.sytes.net/davide/PalladiumWallet/issues/new"));
+    }
+
     // Esc (desktop) or Back (Android) closes the topmost open overlay.
     protected override void OnKeyDown(KeyEventArgs e)
     {