fix(cli): prevent header text artifacts on re-render

Add flexGrow={1} to the text column so it fills the full terminal
width. This ensures Ink clears the entire line when content changes
(e.g. 'awaiting model' → 'llama3.2 (8k)'), preventing leftover
characters from previous renders.

packages/cli/src/tui/components/top-bar.tsx

@@ -77,7 +77,7 @@ export function TopBar({
   return (
     <Box paddingX={1} paddingY={0} marginBottom={1}>
       <MosaicIcon />
-      <Box flexDirection="column">
+      <Box flexDirection="column" flexGrow={1}>
         <Text>
           <Text bold color="#56a0ff">
             Mosaic Stack