fix(tui): F-006 flatten multi-line paste in input render

A multi-line paste put raw newlines into the single-line input slot, overflowing
the fixed-height box and leaving a smeared/stale region. flattenForDisplay collapses
newlines (↵) and tabs for rendering only; the inputBuffer keeps real characters for
submission.

apps/tui-go/internal/app/view.go

@@ -206,6 +206,14 @@ func (m Model) renderMain(h int) string {
 
 // --- input bar ---
 
+// flattenForDisplay collapses control characters (newlines from a multi-line paste, tabs)
+// to a single visible line so the fixed-height input box can't overflow and smear stale cells
+// (F-006). The underlying inputBuffer keeps its real characters for submission.
+func flattenForDisplay(s string) string {
+	repl := strings.NewReplacer("\r\n", "↵ ", "\r", "↵ ", "\n", "↵ ", "\t", " ")
+	return repl.Replace(s)
+}
+
 func (m Model) renderInput() string {
 	t := m.theme
 
@@ -231,7 +239,7 @@ func (m Model) renderInput() string {
 	case m.inputBuffer == "":
 		line = caret + t.span(placeholder, t.P.Faint)
 	default:
-		line = t.span(m.inputBuffer, t.P.FgStrong) + caret
+		line = t.span(flattenForDisplay(m.inputBuffer), t.P.FgStrong) + caret
 	}
 
 	// status sub-line: <session/none> · <uuid> · <mode>