feat(bal): Group D / D1 - configurable distributed calendar reminders + save-only .ics

D1 - number of reminders (default 3, max 5):
  - new NUM_REMINDERS config in plugin_base.py
  - new BalSpinBox widget bound to a BalConfig
  - new pure helper compute_reminder_offsets(days, count): reminders spread
    uniformly across the check-alive period, always before the deadline
    (offset >= 1), at most one per available day, de-duplicated, earliest
    first (e.g. (30,3)->[30,16,1], (2,3)->[2,1], (1,3)->[1], (0,3)->[])
  - create_alarms() rewritten to use it and emit one VALARM per offset with a
    DESCRIPTION reminder text
  - 'Number of reminders' spin box (range 1..5) added to the settings dialog
    and to the Reset list

D1b - save-only .ics (ask where, default to Desktop):
  - BalCalendar.desktop_dir() helper (~/Desktop with home fallback)
  - open_or_save_calendar() no longer opens the file with a calendar app; it
    always shows a 'save as' dialog (.ics filter, default will_event.ics,
    starting on the Desktop) and copies the file there, then confirms.
    Identical behaviour on Windows/Linux/macOS. save_to_cwd -> save_ics_to.

Follow-up: removed the now-unused 'Calendar App' field from the settings
  dialog (and from the Reset list). CALENDAR_APP config and
  open_with_default_app left in place (unused) to avoid unrelated changes.

D2 intentionally skipped (per user request).

tests/test_group_d_alarms.py: 7 new tests (NUM_REMINDERS default/change and
  all distribution rules). Full suite: 217 passed.

bal/core/plugin_base.py

@@ -181,6 +181,14 @@ class BalPlugin(BasePlugin):
         # stay display-only outside the wizard unless the user opts in.
         self.EDITABLE_DATES = BalConfig(config, "bal_editable_dates", False)
 
+        # NUM_REMINDERS (Group D / D1): how many reminder alarms (VALARM) the
+        # exported .ics calendar event should contain. The reminders are spread
+        # uniformly across the check-alive period and always fall BEFORE the
+        # delivery deadline. Default 3; the settings dialog caps it at 5 and the
+        # alarm builder additionally limits it to at most one alarm per available
+        # day.
+        self.NUM_REMINDERS = BalConfig(config, "bal_num_reminders", 3)
+
         self.NO_WILLEXECUTOR = BalConfig(config, "bal_no_willexecutor", True)
         self.HIDE_REPLACED = BalConfig(config, "bal_hide_replaced", True)
         self.HIDE_INVALIDATED = BalConfig(config, "bal_hide_invalidated", True)