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/app_test.go

@@ -988,7 +988,7 @@ func TestRebindingShowsEverywhere(t *testing.T) {
 	if !strings.Contains(scr, "z to reveal") || strings.Contains(scr, "space to reveal") {
 		t.Errorf("detail hint doesn't follow the binding:\n%s", scr)
 	}
-	if !strings.Contains(lastLine(scr), "z reveal") || strings.Contains(lastLine(scr), "reveal/hide") {
+	if !strings.Contains(lastLine(scr), "z show") {
 		t.Errorf("help bar: %q", lastLine(scr))
 	}
 	d.keys("z")