feat: show and hide in the help bar, the same width so it doesn't shift (#24)

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

internal/ui/app/layout_test.go

@@ -136,7 +136,7 @@ func TestLayoutIsSaved(t *testing.T) {
 func TestContextHelpBar(t *testing.T) {
 	t.Parallel()
 	d := newDriver(t)
-	if bar := lastLine(d.screen()); !strings.Contains(bar, "S share") || !strings.HasSuffix(strings.TrimSpace(bar), "? help • q quit") || strings.Contains(bar, "space reveal") {
+	if bar := lastLine(d.screen()); !strings.Contains(bar, "S share") || !strings.HasSuffix(strings.TrimSpace(bar), "? help • q quit") || strings.Contains(bar, "space show") {
 		t.Errorf("Sources help bar: %q", bar)
 	}
 	d.keys("3")