fix: scope list validation to step 3 in compose wizard

[katsar0v]

Problem

When clicking Continue in step 1 of the compose wizard, users see the alert "Please select at least one list" - even though list selection only exists in step 3.

Root Cause

The JavaScript validation used a generic $('form').on('submit') which triggered on ALL forms. Since #mskd-lists-select doesn't exist in step 1, $('#mskd-lists-select').val() returns null, incorrectly triggering the validation.

Fix

Added a check for the list selector's existence before validating:

var $listsSelect = $('#mskd-lists-select');
if ($listsSelect.length > 0) {
    // validate only when element exists
}

Testing

• Step 1: Click Continue → navigates to step 2 (no alert)
• Step 3: Click Send without list → shows validation alert