refactor(ui): remove the viewport navigation bar

Sağ kenardaki dikey orbit/pan/zoom/zoom-extents araç çubuğu tümüyle
kaldırıldı: viewport render'ı, overlay::nav_toolbar fonksiyonu,
show_navbar state + ToggleNavbar mesajı/handler'ı, NAVBAR komutu, View
şeridindeki "Navigation Bar" aracı ve navbar modülü/ikonu. Aynı
gezinme işlemleri komutlar (3DORBIT, PAN, ZOOM*) ve ViewCube üzerinden
erişilebilir durumda.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

assets/icons/navbar.svg

@@ -1,8 +0,0 @@
-<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24" width="64" height="64" fill="none" stroke-linecap="round" stroke-linejoin="round">
-  <circle cx="12" cy="12" r="9" stroke="#e0e0e0" stroke-width="1.5"/>
-  <line x1="12" y1="3" x2="12" y2="8" stroke="#e0e0e0" stroke-width="1.5"/>
-  <line x1="12" y1="16" x2="12" y2="21" stroke="#e0e0e0" stroke-width="1.5"/>
-  <line x1="3" y1="12" x2="8" y2="12" stroke="#e0e0e0" stroke-width="1.5"/>
-  <line x1="16" y1="12" x2="21" y2="12" stroke="#e0e0e0" stroke-width="1.5"/>
-  <text x="9.5" y="7" font-size="4.5" fill="#4cc9f0" stroke="none">N</text>
-</svg>