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[] = [];