/informes TOC below 'Resumen general' — section list with explanations + jump links

Elazar 2026-07-03: on /informes, directly below the 'Resumen general' block, add a table of contents. Each entry = the section name + a one-line explanation of what that section shows + an in-page jump link (anchor) to that section. Cover every major section of the /informes page. Reuse existing anchor ids where sections already have them (e.g. sec-observaciones pattern); add ids where missing. Display/nav only, no data/authz change.