Canonicals aligned with trailing slash, auth-only pages hardened

Canonical URLs across the feedback portal and admin had inconsistent trailing-slash treatment. Some pages emitted the canonical with a slash, others without — the same logical page could look like two distinct URLs to a crawler.

All canonicals now follow a single convention (no trailing slash on leaf paths, slash on directory-style routes) and the response code is consistent. Auth-only pages were also hardened to return a proper 401 instead of a redirect-loop in some edge cases.