validate and deduplicate relay config input in qt gui

Adds validation and deduplication of the relay urls entered in the QT
settings dialog. This is supposed to prevent malformed or duplicated
relay entries.
Also resets the relays to the default value if no (valid) url
is entered. This prevents the user from getting stuck without relays
(otherwise the user would have to research for relay urls manually if
they don't know any).

electrum/gui/qt/settings_dialog.py

@@ -24,7 +24,7 @@
 # SOFTWARE.
 
 import ast
-from typing import TYPE_CHECKING
+from typing import TYPE_CHECKING, Dict
 
 from PyQt6.QtCore import Qt
 from PyQt6.QtWidgets import (QComboBox,  QTabWidget, QDialog, QSpinBox,  QCheckBox, QLabel,
@@ -32,7 +32,7 @@ from PyQt6.QtWidgets import (QComboBox,  QTabWidget, QDialog, QSpinBox,  QCheckB
 
 from electrum.i18n import _, languages
 from electrum import util
-from electrum.util import base_units_list, event_listener
+from electrum.util import base_units_list, event_listener, is_valid_websocket_url
 
 from electrum.gui import messages
 
@@ -181,7 +181,16 @@ class SettingsDialog(QDialog, QtEventListener):
         self.nostr_relays_e = QLineEdit(nostr_relays)
 
         def on_nostr_edit():
-            self.config.NOSTR_RELAYS = str(self.nostr_relays_e.text())
+            relays: Dict[str, None] = dict()  # dicts keep insertion order
+            for url in self.nostr_relays_e.text().split(','):
+                url = url.strip()
+                if url and is_valid_websocket_url(url):
+                    relays[url] = None
+            if relays.keys():
+                self.config.NOSTR_RELAYS = ",".join(relays.keys())
+            else:  # if no valid relays are given, assign default relays from config
+                self.config.NOSTR_RELAYS = None
+            self.nostr_relays_e.setText(self.config.NOSTR_RELAYS)
         self.nostr_relays_e.editingFinished.connect(on_nostr_edit)
 
         msat_cb = checkbox_from_configvar(self.config.cv.BTC_AMOUNTS_PREC_POST_SAT)