Fix markdown streaming duplication by splitting newlines first

- Added string-width library for proper terminal column width calculation
- Fixed wrapLine() to split by newlines before wrapping (like Text component)
- Fixed Loader interval leak by stopping before container removal
- Changed loader message from 'Loading...' to 'Working...'

packages/tui/src/tui.ts

@@ -3,6 +3,7 @@
  */
 
 import type { Terminal } from "./terminal.js";
+import { visibleWidth } from "./utils.js";
 
 /**
  * Component interface - all components must implement this
@@ -21,6 +22,8 @@ export interface Component {
 	handleInput?(data: string): void;
 }
 
+export { visibleWidth };
+
 /**
  * Container - a component that contains other components
  */
@@ -211,6 +214,9 @@ export class TUI extends Container {
 		// Render from first changed line to end
 		for (let i = firstChanged; i < newLines.length; i++) {
 			if (i > firstChanged) buffer += "\r\n";
+			if (visibleWidth(newLines[i]) > width) {
+				throw new Error("Rendered line exceeds terminal width");
+			}
 			buffer += newLines[i];
 		}