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/gui/qt/calendar.py

@@ -30,6 +30,25 @@ class BalCalendar:
             _logger.error(f"starting calendar app {e}")
             return False
 
+    @staticmethod
+    def desktop_dir():
+        """Return the user's Desktop directory (Group D / D1b).
+
+        Used as the initial folder of the "save .ics" dialog. On Windows this is
+        normally ``C:\\Users\\<name>\\Desktop``; on Linux/macOS ``~/Desktop`` is
+        used when it exists. If the Desktop cannot be located the home directory
+        is returned as a safe fallback, so the save dialog always opens
+        somewhere sensible.
+
+        Returns:
+            An absolute directory path (string).
+        """
+        home = os.path.expanduser("~")
+        desktop = os.path.join(home, "Desktop")
+        if os.path.isdir(desktop):
+            return desktop
+        return home
+
 
     @staticmethod
     def format_time(time):