LMArena migrated to @supabase/ssr chunked auth cookies: the single
arena-auth-prod-v1 cookie is now empty and the session is split across
arena-auth-prod-v1.0, .1, … (ascending). Pasting the now-empty single
cookie sent an empty session, which upstream rejected as "invalid cookie".
reconstructLMArenaCookie() rebuilds the single cookie from its chunks
(ascending join, no decode/parse — combineChunks semantics), preserving
the rest of the pasted jar; a non-empty single cookie is forwarded
unchanged (back-compat). The credential UX now instructs pasting the full
Cookie header and tracks the .0/.1 storage keys.
Closes#4271
5cca4ff90c