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/utils.ts

@@ -0,0 +1,12 @@
+import stringWidth from "string-width";
+
+/**
+ * Calculate the visible width of a string in terminal columns.
+ * This correctly handles:
+ * - ANSI escape codes (ignored)
+ * - Emojis and wide characters (counted as 2 columns)
+ * - Combining characters (counted correctly)
+ */
+export function visibleWidth(str: string): number {
+	return stringWidth(str);
+}