The Sign-in page uses master page called blank.html. You can find it here: /ow_themes/origin/master_pages/blank.html.
If you want the bottom menu to be shown on the Sign-in page you should add the component {$bottom_menu} to this master page and them recompile resource by enabling DEV_MODE.
Be advised, this master page is used for all blank page like: forgot password, verification email, etc. So, if you add the component to this mater page, the bottom menu will be shown on all these pages.