fix: handle dynamic dimensions in Math output shape

internal/captcha/handler.go

@@ -343,10 +343,31 @@ func preprocessMath(img image.Image) ([]float32, error) {
 func decodeMath(output []float32, outputShape []int64) string {
 	// 输出格式: [num_chars, batch, timesteps] = [51, 1, 19]
 	// 需要转置为 [batch, timesteps, num_chars] = [1, 19, 51]
+	if len(outputShape) < 3 || len(output) == 0 {
+		return ""
+	}
+
 	numChars := int(outputShape[0])  // 51
 	batch := int(outputShape[1])     // 1
 	timesteps := int(outputShape[2]) // 19
 
+	// 处理动态维度（可能返回 -1）
+	if numChars <= 0 {
+		numChars = 51 // 默认字符数
+	}
+	if batch <= 0 {
+		batch = 1
+	}
+	if timesteps <= 0 {
+		// 从输出大小推断
+		timesteps = len(output) / (numChars * batch)
+	}
+
+	// 边界检查
+	if numChars <= 0 || batch <= 0 || timesteps <= 0 {
+		return ""
+	}
+
 	// 转置: output[c*batch*t + b*t + t] -> transposed[b*timesteps*numChars + t*numChars + c]
 	transposed := make([]float32, batch*timesteps*numChars)
 	for b := 0; b < batch; b++ {