Refactor LeftBar/RightBarContent into main-site and subdomain variants.
Subdomain sites render a simplified, brand-colored shell iterating a new
pure nav-config module (NAV_CONFIG), with a "back to freno.me" affordance
and inline SVG icon resolver. The main site retains its bespoke bars
(Recent Posts, auth-aware nav, admin links, widgets) unchanged.
Add nav-config.ts as an import-free, unit-testable module defining the
per-site link sets (with external/href/auth-gating/icon contract), plus
filterNavByAuth helper and BACK_TO_FRENO link.
