fix: handle MAL API returning numbers instead of strings for zero-value statistics

The MyAnimeList API inconsistently returns statistics status fields (watching,
completed, on_hold, dropped, plan_to_watch) as quoted strings for non-zero
values (e.g. "8217") but as bare numbers for zero values (e.g. 0). This caused
JSON unmarshal errors for anime with zero counts in any status field.

Introduce a FlexString custom type that implements json.Unmarshaler to accept
both JSON strings and JSON numbers, always storing the result as a string. The
type definition lives in MALTypes.go and the unmarshal logic in MALFunctions.go
to keep static types and behavior separate.

MALFunctions.go

@@ -11,6 +11,21 @@ import (
 	"strings"
 )
 
+// Unmarshalling accidental numbers received from MAL to strings
+func (f *FlexString) UnmarshalJSON(data []byte) error {
+	var s string
+	if err := json.Unmarshal(data, &s); err == nil {
+		*f = FlexString(s)
+		return nil
+	}
+	var n json.Number
+	if err := json.Unmarshal(data, &n); err == nil {
+		*f = FlexString(string(n))
+		return nil
+	}
+	return fmt.Errorf("FlexString: invalid value")
+}
+
 func MALHelper(method string, malUrl string, body url.Values) (json.RawMessage, string, error) {
 	client := &http.Client{}