feat(server): space avatars and full member profiles in the structure response

server/src/WebDrop.Core/AvatarData.cs

@@ -0,0 +1,26 @@
+namespace WebDrop.Core;
+
+/// <summary>头像(账号头像、空间头像)的 data URL 校验。浏览器里已经裁剪好并重新编码,服务端只做格式、大小和文件头检查</summary>
+public static class AvatarData
+{
+    /// <summary>静态头像(256×256 WebP/PNG/JPEG)的长度上限(字符数)</summary>
+    public const int MaxStaticChars = 350_000;
+    /// <summary>账号动图头像保留全部帧,约 4.5 MB</summary>
+    public const int MaxAccountGifChars = 6_000_000;
+    /// <summary>空间头像会跟着空间信息发给所有成员和空间列表,动图上限比账号头像小,约 1.5 MB</summary>
+    public const int MaxSpaceGifChars = 2_000_000;
+
+    private static readonly string[] Prefixes = ["data:image/png;base64,", "data:image/jpeg;base64,", "data:image/webp;base64,", "data:image/gif;base64,"];
+
+    /// <summary>不合法时抛 BadRequest</summary>
+    public static void Validate(string avatar, int maxGifChars)
+    {
+        var prefix = Prefixes.FirstOrDefault(p => avatar.StartsWith(p, StringComparison.Ordinal));
+        var gif = prefix?.Contains("gif", StringComparison.Ordinal) == true;
+        if (prefix is null || avatar.Length > (gif ? maxGifChars : MaxStaticChars) || avatar.Length <= prefix.Length) throw new WebDropException(ErrorCode.BadRequest);
+        byte[] bytes;
+        try { bytes = Convert.FromBase64String(avatar[prefix.Length..]); }
+        catch (FormatException) { throw new WebDropException(ErrorCode.BadRequest); }
+        if (gif && !(bytes.Length > 6 && bytes[0] == (byte)'G' && bytes[1] == (byte)'I' && bytes[2] == (byte)'F')) throw new WebDropException(ErrorCode.BadRequest);
+    }
+}