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/manifest.json

@@ -1,7 +1,7 @@
 {
     "name": "bal",
     "fullname": "Bitcoin After Life",
-    "version": "0.3.6",
+    "version": "0.3.7",
     "description": "Provides free and decentralized Bitcoin inheritance support. Build time-locked 'will' transactions that transfer funds to your heirs if you stop refreshing them (dead-man's switch), optionally relayed by will-executor servers.",
     "author": "Svatantrya",
     "licence": "MIT",