Friendly retry copy for dropped mobile navigation fetches

client/unhandled-error 'Load failed' hits the global handler instead of a friendly Spanish retry affordance when a nav fetch (e.g. back to /practicas after a review) drops on mobile WebKit. 13 occurrences/30 days, 10 distinct users, 5 routes, mobile-only, no data loss (the underlying action, e.g. review submit, already succeeded) - it's a naked TypeError shown to users, not a defect. Scope: add a handled path with a 'revisá tu conexión' retry affordance for dropped navigation fetches, per the no-silent-failures contract.