fix: 修复 POP 签名 percentEncode 使用 Unicode 字母集导致非 ASCII 字符未编码的问题

CharacterSet.alphanumerics 包含所有 Unicode 字母（阿拉伯文等），导致搜索含非 ASCII 关键词时签名不匹配。改为仅允许 ASCII 字母数字加 -_.~

apps/macos/VODManager/Sources/VODManager/Services/VODSignature.swift

@@ -40,8 +40,7 @@ struct VODSignature {
     }
 
     private static func percentEncode(_ value: String) -> String {
-        var allowed = CharacterSet.alphanumerics
-        allowed.insert(charactersIn: "-_.~")
+        let allowed = CharacterSet(charactersIn: "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_.~")
         return value.addingPercentEncoding(withAllowedCharacters: allowed) ?? value
     }
 }