Warn when saving unevenly costs the family days

Only ninety-six days survive a child's fourth birthday, and with two parents
that is forty-eight each rather than a shared pool. A parent sitting on sixty
days while the other has ten therefore forfeits twelve that the family could
have kept, and nothing said so.

The advice leads with taking the days as leave before the birthday, which
turns them into actual time at home, and offers transferring only as the
fallback that merely preserves them. The transferable figure is bounded by
what Försäkringskassan actually reports as transferable, since the reserved
days cannot move, and the copy says plainly that a transfer happens there and
not in this app. Without the partner's figures the tip hedges rather than
inventing a number.

backend/app/config.py

@@ -41,6 +41,12 @@ class LeaveRules(BaseModel):
     max_savable_days_per_parent_at_age_4: int = 48
     hard_expiry_age_years: int = 12
 
+    # A couple's per-parent age-4 pooling-loss tip (see
+    # `app.utils.tip_predicates.age4_pooling_loss`) only starts warning once
+    # the child has turned this many years old — the first couple of years
+    # are too early to usefully act on, so don't nag before then.
+    age4_pooling_tip_min_child_age_years: int = 2
+
     sgi_protected_first_months: int = 12
     sgi_min_days_per_week_after_age_1: int = 5