feat: review and rebalance Week Ahead (Closes #1188)

src/today_store.py

@@ -495,6 +495,7 @@ class TodayStore:
             raise ValueError("timezone must be a valid IANA timezone") from None
         normalized = []
         seen_dates = set()
+        seen_ids = set()
         for day in days:
             if not isinstance(day, dict):
                 raise ValueError("each Week Ahead date must be an object")
@@ -511,6 +512,10 @@ class TodayStore:
                 ids=day.get("ids", []), capacity_minutes=day.get("capacity_minutes"),
                 estimates=day.get("estimates", {}), plan_date=plan_date, timezone=timezone,
             )
+            duplicate_ids = seen_ids.intersection(plan["ids"])
+            if duplicate_ids:
+                raise ValueError("work must be assigned to only one Week Ahead date")
+            seen_ids.update(plan["ids"])
             plan.pop("timezone")
             normalized.append(plan)
         normalized.sort(key=lambda item: item["plan_date"])