fix: prevent stuck launch dialog when history file is missing

update_history_file() opened dzg.history in read mode without handling a
missing file. dzg.history is only created the first time it is written, so
on a fresh install the very first server connection raised FileNotFoundError
inside the post-launch cleanup callback:

  _destroy_on_idle -> _add_to_history_and_return -> update_history_file

The exception propagated out of the GLib idle callback before destroy_dialog()
ran, orphaning the "Waiting for DayZ to launch" modal so it could never be
closed from the UI and its Cancel button was unresponsive.

Treat a missing history file as an empty history so the first connection
records history normally instead of crashing the cleanup callback.

dzgui/managers/config.py

@@ -55,8 +55,11 @@ class ConfigManager:
         self.update_config(Preferences.IP_LIST, ips)
 
     def update_history_file(self, fqip: str) -> None:
-        with open(self.prefs.paths.history, "r") as f:
-            ips = [line.rstrip() for line in f.readlines()]
+        try:
+            with open(self.prefs.paths.history, "r") as f:
+                ips = [line.rstrip() for line in f.readlines()]
+        except FileNotFoundError:
+            ips = []
 
         seen = set()
         ips.append(fqip)