Release 1.13.0

Accessibility (roadmap WS-T task 3), from an axe survey of every site. The navbar's Sign In button read at 3.67:1 (white on #3b82f6) on nine sites: every theme now derives --accent-strong / --on-accent-strong, the accent deepened (under white) or lightened (under dark ink) just enough for 4.5:1, and the button uses them (test/login-contrast.test.js checks all 35 themes). The two tokens are in theme-sync and the loader's map. The footer reads { label, href } items as { name, url }, so a site passing the navbar's shape no longer renders empty links (Media and Games did). Additive.