feat(mobile-nav): respect Chatwoot brand color and navigate via SPA router

Two improvements to the mobile bottom navigation script:

- Brand color: the nav background was hardcoded to #29a2a7 and ignored the
  brand color configured in the Chatwoot super admin. It now reads the brand
  color from window.globalConfig / window.chatwootConfig (BRAND_COLOR), with an
  optional window.MEGA_MOBILE_NAV_COLOR override and the previous color kept as
  fallback, so each install matches its own theme automatically.

- SPA navigation: tapping a nav item called window.location.assign(), which
  forced a full reload of the whole Chatwoot app on every navigation. It now
  navigates through the SPA router (history.pushState + popstate, which
  vue-router listens to), falling back to a full navigation only on error.