fix(publisher): render math server-side via KaTeX

fix(publisher): footer credit links to the editor Space

feat(publisher): editor pill shows who you're signed in as

feat(publisher): editor quick-link for signed-in editors

fix(ui,publisher): kill theme/color flickers and translation prompts

refactor(backend/publisher): transformer pipeline + shared registry + shiki highlighting

refactor: remove MUI, unify publisher pipeline, add shared component registry