Add admin panel: visit counter, product editor, screenshots upload

internal/handlers/templates/admin_nav.html

@@ -0,0 +1,21 @@
+{{define "admin_nav"}}
+<header class="admin-header">
+    <div class="container admin-header__inner">
+        <a href="/admin/" class="logo">
+            <span class="logo__icon">◆</span>
+            <span class="logo__text">Shop Admin</span>
+        </a>
+        <nav class="admin-nav">
+            <a href="/admin/" class="admin-nav__link">Главная</a>
+            <a href="/admin/products" class="admin-nav__link">Товары</a>
+            <a href="/admin/products/new" class="admin-nav__link admin-nav__link--accent">+ Добавить</a>
+        </nav>
+        <div class="admin-header__user">
+            <span>{{.Email}}</span>
+            <form method="POST" action="/admin/logout">
+                <button type="submit" class="btn btn--ghost btn--sm">Выйти</button>
+            </form>
+        </div>
+    </div>
+</header>
+{{end}}