Rework mobile footer UX (Nick request, #commongrid thread 1779100357.769149)

Nick flagged the mobile footer as cramped/poorly laid out. Diagnosis: 4-col desktop grid collapsed to 2-col below 820px, brand block was 2fr, headings landed unpredictably. Fix shipped in PR #254: mobile <=560px = vertical stack with dividers and 32px tap targets; tablet 561-820px = full-width brand row + 3 equal link cols; desktop unchanged. Also moved brand link inline styles into .foot-brand-link class. Branch: meridian/mobile-footer-redesign PR: https://github.com/TextureHQ/commongrid/pull/254