config: allow setting "forgetconfig" option in file, not just on CLI

- by specifying "default=<VALUE>" in commands.py, <VALUE> would always overwrite what is in the config file
- note `$ ./run_electrum -o setconfig forget_config true` still does not work,
  as that first sets forget_config in memory, and then - by virtue of the setting -
  it refuses to write changes to the config file
  - hence this option would have to be set manually by editing the json
    (or as a CLI flag, as before)

ref https://github.com/spesmilo/electrum/pull/10421#issuecomment-3765862081

electrum/commands.py

@@ -2397,7 +2397,7 @@ def add_global_options(parser, suppress=False):
         "--rpcpassword", dest=SimpleConfig.RPC_PASSWORD.key(), default=argparse.SUPPRESS,
         help=argparse.SUPPRESS if suppress else "RPC password")
     group.add_argument(
-        "--forgetconfig", action="store_true", dest=SimpleConfig.CONFIG_FORGET_CHANGES.key(), default=False,
+        "--forgetconfig", action="store_true", dest=SimpleConfig.CONFIG_FORGET_CHANGES.key(), default=None,
         help=argparse.SUPPRESS if suppress else "Forget config on exit")
 
 

electrum/simple_config.py

@@ -446,6 +446,7 @@ class SimpleConfig(Logger):
 
     def save_user_config(self):
         if self.CONFIG_FORGET_CHANGES:
+            self.logger.warning(f"not saving config changes to disk as {self.cv.CONFIG_FORGET_CHANGES.key()} is set", only_once=True)
             return
         if not self.path:
             return