feat(savings): Add savings module with categories, transactions, recurring plans

- Categories: regular, deposits, credits, recurring, multi-user, accounts
- Transactions: deposits and withdrawals with user tracking
- Recurring plans: monthly payment obligations per user
- Stats: overdues calculation with allocation algorithm
- Excludes is_account categories from total sums
- Documentation: docs/SAVINGS.md

internal/handler/habits.go

@@ -146,6 +146,14 @@ func (h *HabitHandler) Log(w http.ResponseWriter, r *http.Request) {
 			writeError(w, "habit not found", http.StatusNotFound)
 			return
 		}
+		if errors.Is(err, service.ErrFutureDate) {
+			writeError(w, "cannot log habit for future date", http.StatusBadRequest)
+			return
+		}
+		if errors.Is(err, service.ErrAlreadyLogged) {
+			writeError(w, "habit already logged for this date", http.StatusConflict)
+			return
+		}
 		writeError(w, "failed to log habit", http.StatusInternalServerError)
 		return
 	}