fix(tui): share composer prompt gap metric

Use one exported prompt gap constant for both composer width math and prompt prefix rendering.

ui-tui/src/lib/inputMetrics.ts

@@ -1,5 +1,7 @@
 import { stringWidth } from '@hermes/ink'
 
+export const COMPOSER_PROMPT_GAP_WIDTH = 1
+
 let _seg: Intl.Segmenter | null = null
 const seg = () => (_seg ??= new Intl.Segmenter(undefined, { granularity: 'grapheme' }))
 
@@ -54,7 +56,7 @@ export function inputVisualHeight(value: string, columns: number) {
 }
 
 export function composerPromptWidth(promptText: string) {
-  return Math.max(1, stringWidth(promptText)) + 1
+  return Math.max(1, stringWidth(promptText)) + COMPOSER_PROMPT_GAP_WIDTH
 }
 
 export function stableComposerColumns(totalCols: number, promptWidth: number) {