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/tui/src/components/markdown.ts

@@ -103,8 +103,11 @@ export class Markdown implements Component {
 			return result;
 		}
 
+		// Replace tabs with 3 spaces for consistent rendering
+		const normalizedText = this.text.replace(/\t/g, "   ");
+
 		// Parse markdown to HTML-like tokens
-		const tokens = marked.lexer(this.text);
+		const tokens = marked.lexer(normalizedText);
 
 		// Convert tokens to styled terminal output
 		const renderedLines: string[] = [];

packages/tui/src/components/text.ts

@@ -63,8 +63,11 @@ export class Text implements Component {
 			return result;
 		}
 
+		// Replace tabs with 3 spaces for consistent rendering
+		const normalizedText = this.text.replace(/\t/g, "   ");
+
 		const lines: string[] = [];
-		const textLines = this.text.split("\n");
+		const textLines = normalizedText.split("\n");
 
 		for (const line of textLines) {
 			// Measure visible length (strip ANSI codes)

packages/tui/src/utils.ts

@@ -6,7 +6,10 @@ import stringWidth from "string-width";
  * - ANSI escape codes (ignored)
  * - Emojis and wide characters (counted as 2 columns)
  * - Combining characters (counted correctly)
+ * - Tabs (replaced with 3 spaces for consistent width)
  */
 export function visibleWidth(str: string): number {
-	return stringWidth(str);
+	// Replace tabs with 3 spaces before measuring
+	const normalized = str.replace(/\t/g, "   ");
+	return stringWidth(normalized);
 }