This file contains the sidebar section to be added.
Replace the section between "{{-- Warehouse with sub-menu --}}" and "{{-- Remaining nav items --}}" 
with the following additional nav item added AFTER the warehouse section:

        {{-- Letters --}}
        @php
            $isLettersActive = request()->routeIs('letters.*');
        @endphp
        <a href="{{ route('letters.index') }}"
           class="flex items-center gap-3 px-3 py-2.5 rounded-lg text-sm font-medium transition-colors duration-150
           {{ $isLettersActive ? 'bg-teal-50 text-teal-700 border border-teal-200' : 'text-gray-600 hover:bg-gray-100 hover:text-gray-900' }}">
            <svg class="w-5 h-5 flex-shrink-0" fill="none" stroke="currentColor" viewBox="0 0 24 24">
                <path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M3 8l7.89 5.26a2 2 0 002.22 0L21 8M5 19h14a2 2 0 002-2V7a2 2 0 00-2-2H5a2 2 0 00-2 2v10a2 2 0 002 2z"/>
            </svg>
            <span>{{ __('nav.letters') }}</span>
        </a>