Fix /administrar dup-title+stale-sidebar bug; extend header/breadcrumb pattern fleet-wide across admin section

Elazar direct, follow-up to MARS-448: (1) BUG /administrar shows two titles (old title element left in place alongside new PageHeader title) and the sidebar nav label was not updated to match the new 'Panel Administración' page title. Fix both. (2) SCOPE EXPANSION: /administrar/usuarios already has the title+breadcrumb-links+back-arrow pattern and Elazar accepts it as the reference (grumbled about the visual style but confirmed keep it). /docentes (e.g. ?periodo=... query variant) does NOT have this pattern. Elazar wants ALL admin-section pages to carry title+breadcrumb-links+back-arrow consistently -- inventory the admin route tree, find every page missing the pattern (docentes confirmed missing, check the rest), apply the same header/breadcrumb component used on administrar/usuarios everywhere it applies.