Fix tab rendering in TUI components

Replace tabs with 3 spaces for consistent rendering and width calculation:
- Updated visibleWidth() to normalize tabs before measuring
- Updated Text and Markdown components to replace tabs when rendering
- Updated tool-execution display for read/write tools to replace tabs

This fixes background color rendering issues when displaying files with tab indentation.

packages/coding-agent/src/tui/tool-execution.ts

@@ -13,6 +13,13 @@ function shortenPath(path: string): string {
 	return path;
 }
 
+/**
+ * Replace tabs with spaces for consistent rendering
+ */
+function replaceTabs(text: string): string {
+	return text.replace(/\t/g, "   ");
+}
+
 /**
  * Generate a unified diff between old and new strings with line numbers
  */
@@ -120,7 +127,7 @@ export class ToolExecutionComponent extends Container {
 				const displayLines = lines.slice(0, maxLines);
 				const remaining = lines.length - maxLines;
 
-				text += "\n\n" + displayLines.map((line: string) => chalk.dim(line)).join("\n");
+				text += "\n\n" + displayLines.map((line: string) => chalk.dim(replaceTabs(line))).join("\n");
 				if (remaining > 0) {
 					text += chalk.dim(`\n... (${remaining} more lines)`);
 				}
@@ -142,7 +149,7 @@ export class ToolExecutionComponent extends Container {
 				const displayLines = lines.slice(0, maxLines);
 				const remaining = lines.length - maxLines;
 
-				text += "\n\n" + displayLines.map((line: string) => chalk.dim(line)).join("\n");
+				text += "\n\n" + displayLines.map((line: string) => chalk.dim(replaceTabs(line))).join("\n");
 				if (remaining > 0) {
 					text += chalk.dim(`\n... (${remaining} more lines)`);
 				}