fix: esconder toolbar duplicada do viewer — controlos na barra principal

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: nelsonduarte

app/viewer/panel.py

@@ -84,6 +84,8 @@ def _nav_btn(icon_name):
                   self._zoom_in_btn, self._fit_btn,
                   self._prev_btn, self._page_lbl, self._next_btn):
             hdr_lay.addWidget(w)
+        self._hdr = hdr
+        self._hdr.setVisible(False)
         layout.addWidget(hdr)
 
         # ── Placeholder ─────────────────────────────────────────────────────