feat(gui): custom login page, session-based auth

Replace nginx auth_basic + HTTP Basic Auth with a styled Flask login form.
- Session-based authentication (cookie, session.permanent)
- Custom login page with logo, error state, clean form design
- CSRF check skipped for /login route
- Logout button in sidebar footer
- nginx auth_basic removed; ADMIN_PASSWORD restored in .env

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

gui/templates/base.html

@@ -26,6 +26,12 @@
         Скрипты
       </a>
     </nav>
+    <div class="sidebar-footer">
+      <a href="{{ url_for('logout') }}" class="logout-btn">
+        <svg width="16" height="16" viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2"><path d="M9 21H5a2 2 0 0 1-2-2V5a2 2 0 0 1 2-2h4"/><polyline points="16 17 21 12 16 7"/><line x1="21" y1="12" x2="9" y2="12"/></svg>
+        Выйти
+      </a>
+    </div>
   </aside>
   <div class="layout">
     <main>