fix: correct total trial count when adding additional trials (#385)

When a study is cancelled mid-way and the user selects 'Run additional
trials', settings.n_trials was incremented by n_additional_trials,
accumulating the original total into the new count. E.g. cancelling 200
trials at 30 and adding 10 gave n_trials=210 instead of 40, causing
'Running trial 31 of 210...' and planning 180 more trials instead of 10.

Fix by recalculating n_trials from actual completed trials + additional,
so the total reflects the new intended target, not the old one.

Fixes #379

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

src/heretic/main.py

@@ -802,7 +802,7 @@ def run():
                     if n_additional_trials == 0:
                         continue
 
-                    settings.n_trials += n_additional_trials
+                    settings.n_trials = len(study.trials) + n_additional_trials
                     study.set_user_attr("settings", settings.model_dump_json())
                     study.set_user_attr("finished", False)