AddTrackModal - placeholder input

ref #11

src/components/AddTrackModal.svelte

@@ -100,6 +100,7 @@
                     for="trackname"
                     class="block text-sm font-medium text-gray-700">{$_('track-name')}</label>
                   <input
+                    placeholder={$_('track-name')}
                     class:border-red-500={isTracknameValid}
                     class:focus:border-red-500={isTracknameValid}
                     class:focus:ring-red-500={isTracknameValid}