qt: fix whitepaper URL and use xdg-open for web links on Linux

- update whitepaper URL to github.com/palladium-coin/whitepaper
- replace webbrowser.open() with xdg-open on Linux to ensure the
  desktop browser is used instead of text-based alternatives (w3m)

electrum/gui/qt/main_window.py

@@ -845,7 +845,7 @@ class ElectrumWindow(QMainWindow, MessageBoxMixin, Logger, QtEventListener):
         self.help_menu.addSeparator()
         self.help_menu.addAction(_("&Documentation"), lambda: webopen("https://github.com/palladium-coin/pallectrum/blob/main/user-guide.md")).setShortcut(QKeySequence.StandardKey.HelpContents)
         if not constants.net.TESTNET:
-            self.help_menu.addAction(_("&White Paper"), lambda: webopen("https://github.com/palladium-coin/palladiumcore/blob/master/assets/whitepaper/(original)whitepaper.pdf"))
+            self.help_menu.addAction(_("&White Paper"), lambda: webopen("https://github.com/palladium-coin/whitepaper/blob/main/whitepaper.pdf"))
         self.help_menu.addAction(_("&Report Bug"), self.show_report_bug)
         self.help_menu.addSeparator()
         self.help_menu.addAction(_("&Donate to server"), self.donate_to_server)

electrum/gui/qt/util.py

@@ -1337,14 +1337,17 @@ def font_height(widget: QWidget = None) -> int:
 
 
 def webopen(url: str):
-    if sys.platform == 'linux' and os.environ.get('APPIMAGE'):
-        # When on Linux webbrowser.open can fail in AppImage because it can't find the correct libdbus.
-        # We just fork the process and unset LD_LIBRARY_PATH before opening the URL.
-        # See #5425
-        if os.fork() == 0:
-            del os.environ['LD_LIBRARY_PATH']
-            webbrowser.open(url)
-            os._exit(0)
+    if sys.platform == 'linux':
+        if os.environ.get('APPIMAGE'):
+            # When on Linux webbrowser.open can fail in AppImage because it can't find the correct libdbus.
+            # We just fork the process and unset LD_LIBRARY_PATH before opening the URL.
+            # See #5425
+            if os.fork() == 0:
+                del os.environ['LD_LIBRARY_PATH']
+                os.system(f'xdg-open "{url}" &')
+                os._exit(0)
+        else:
+            os.system(f'xdg-open "{url}" &')
     else:
         webbrowser.open(url)