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.

MALTypes.go

@@ -1,6 +1,10 @@
 package main
 
-import "time"
+import (
+	"time"
+)
+
+type FlexString string
 
 type MyAnimeListJWT struct {
 	TokenType    string `json:"token_type"`
@@ -129,11 +133,11 @@ type MALAnime struct {
 	Statistics struct {
 		NumListUsers int `json:"num_list_users" ts_type:"numListUsers"`
 		Status       struct {
-			Watching    string `json:"watching" ts_type:"watching"`
-			Completed   string `json:"completed" ts_type:"completed"`
-			OnHold      string `json:"on_hold" ts_type:"onHold"`
-			Dropped     string `json:"dropped" ts_type:"dropped"`
-			PlanToWatch string `json:"plan_to_watch" ts_type:"planToWatch"`
+			Watching    FlexString `json:"watching" ts_type:"string"`
+			Completed   FlexString `json:"completed" ts_type:"string"`
+			OnHold      FlexString `json:"on_hold" ts_type:"string"`
+			Dropped     FlexString `json:"dropped" ts_type:"string"`
+			PlanToWatch FlexString `json:"plan_to_watch" ts_type:"string"`
 		}
 	}
 }