修复RichTextBox编译错误

- 将txtLog从TextBox改为RichTextBox以支持彩色文本
- 更新ScrollBars枚举为RichTextBoxScrollBars
- 解决SelectionColor属性不存在的编译错误

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Form1.Designer.cs

@@ -59,7 +59,7 @@ namespace SlideCombine
 
             grpProgress = new GroupBox();
             progressBar = new ProgressBar();
-            txtLog = new TextBox();
+            txtLog = new RichTextBox();
 
             // 设置源文件夹组
             grpSourceFolder.Text = "📁 PDF文件夹路径";
@@ -180,7 +180,7 @@ namespace SlideCombine
             txtLog.Location = new Point(15, 60);
             txtLog.Size = new Size(640, 200);
             txtLog.Multiline = true;
-            txtLog.ScrollBars = ScrollBars.Vertical;
+            txtLog.ScrollBars = RichTextBoxScrollBars.Vertical;
             txtLog.ReadOnly = true;
             txtLog.Font = new Font("Consolas", 9F, FontStyle.Regular);
             txtLog.BackColor = Color.FromArgb(248, 248, 248);
@@ -212,6 +212,6 @@ namespace SlideCombine
         private Button btnExit;
         private GroupBox grpProgress;
         private ProgressBar progressBar;
-        private TextBox txtLog;
+        private RichTextBox txtLog;
     }
 }