feat(bal): export calendar .ics as separate reminder events (v0.3.7)

- open_or_save_calendar(): emit one VEVENT per reminder instead of a single
  event with VALARM blocks. Dates come from compute_reminder_offsets(), spread
  across the check-alive period, with the LAST event one day before the
  locktime. Unique UID per event (bal-<wallet>-<offset>d) and numbered
  summaries ' (reminder N/total)'.
- Removed the now-unused create_alarms() method (no more VALARM).
- Default save filename changed from will_event.ics to BAL_will_event.ics.
- common.py: added timedelta to the datetime import.
- plugin_base.py: updated the NUM_REMINDERS comment for the new behaviour.
- tests: replaced the VALARM E1 test with test_e1_build_separate_events_for_giovanna.
- Bumped version to 0.3.7 (4 files) and added CHANGELOG entry #13.

Tests: 239 passed. ruff: no new errors.

bal/gui/qt/common.py

@@ -22,7 +22,7 @@ import subprocess
 import tempfile
 import time
 import traceback
-from datetime import datetime, timezone
+from datetime import datetime, timedelta, timezone
 from decimal import Decimal
 from functools import partial
 from typing import Any, Callable, Mapping, Optional, Union