Fix GTK file chooser to show network locations on Linux (#19855)

GTK file choosers default to local_only=TRUE, which hides network
locations (SMB, SFTP, NFS) mounted via GVFS. This change calls
gtk_file_chooser_set_local_only(FALSE) to enable network browsing.

Fixes browsing to network shares in OpenFileDialog, OpenFolderDialog,
and SaveFileDialog on Linux.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: meywd <meywd@users.noreply.github.com>
Co-authored-by: Claude <noreply@anthropic.com>

src/Avalonia.X11/NativeDialogs/Gtk.cs

@@ -61,7 +61,10 @@ namespace Avalonia.X11.NativeDialogs
 
         [DllImport(GtkName)]
         public static extern void gtk_file_chooser_set_select_multiple(IntPtr chooser, bool allow);
-        
+
+        [DllImport(GtkName)]
+        public static extern void gtk_file_chooser_set_local_only(IntPtr chooser, bool local_only);
+
         [DllImport(GtkName)]
         public static extern void gtk_file_chooser_set_do_overwrite_confirmation(IntPtr chooser, bool do_overwrite_confirmation);
 

src/Avalonia.X11/NativeDialogs/GtkNativeFileDialogs.cs

@@ -87,6 +87,7 @@ namespace Avalonia.X11.NativeDialogs
             }
 
             gtk_window_set_modal(dlg, true);
+            gtk_file_chooser_set_local_only(dlg, false);
             var tcs = new TaskCompletionSource<string[]?>();
             List<IDisposable>? disposables = null;