限制素材分离图片的 Base64 解码大小

对源图和服务端处理图在解码前后统一执行 64 MiB 上限检查，避免无界内存分配。

apps/ai-game-creator-shell/src-tauri/src/ui_editor/commands/separation/workflow/extract.rs

@@ -5,6 +5,27 @@ use std::path::PathBuf;
 use std::time::Instant;
 use std::{fs, io::Cursor};
 
+const MAX_IMAGE_PAYLOAD_BYTES: usize = 64 * 1024 * 1024;
+
+fn decode_bounded_base64(value: &str, label: &str) -> Result<Vec<u8>, String> {
+    if value.len() > (MAX_IMAGE_PAYLOAD_BYTES / 3) * 4 + 4 {
+        return Err(format!(
+            "{label}超过 {} MiB 字节上限",
+            MAX_IMAGE_PAYLOAD_BYTES / 1024 / 1024
+        ));
+    }
+    let decoded = base64::engine::general_purpose::STANDARD
+        .decode(value.trim())
+        .map_err(|error| format!("解码{label}失败：{error}"))?;
+    if decoded.len() > MAX_IMAGE_PAYLOAD_BYTES {
+        return Err(format!(
+            "{label}超过 {} MiB 字节上限",
+            MAX_IMAGE_PAYLOAD_BYTES / 1024 / 1024
+        ));
+    }
+    Ok(decoded)
+}
+
 #[derive(Deserialize)]
 struct RawEditResponse {
     data: Vec<RawEditItem>,
@@ -54,9 +75,7 @@ async fn raw_extract_inner(
         .strip_prefix("data:")
         .and_then(|value| value.strip_suffix(";base64"));
     let is_png = mime.is_some_and(|value| value.eq_ignore_ascii_case("image/png"));
-    let image_bytes = base64::engine::general_purpose::STANDARD
-        .decode(data.trim())
-        .map_err(|error| format!("解码源图失败：{error}"))?;
+    let image_bytes = decode_bounded_base64(data, "源图")?;
     if image_bytes.is_empty() {
         return Err("源图不能为空".to_string());
     }
@@ -199,9 +218,7 @@ async fn write_processed_image_inner(
             .split_once(',')
             .map(|(_, data)| data)
             .ok_or_else(|| "处理图 data URL 无效".to_string())?;
-        let processed_bytes = base64::engine::general_purpose::STANDARD
-            .decode(encoded)
-            .map_err(|error| format!("解析处理图失败：{error}"))?;
+        let processed_bytes = decode_bounded_base64(encoded, "处理图")?;
         let dimensions = image::ImageReader::new(Cursor::new(processed_bytes.as_slice()))
             .with_guessed_format()
             .map_err(|error| format!("识别处理图格式失败：{error}"))?