The offline auto-detect shipped in #202 (cold-start no-connection dialog / auto
offline) and #203 (mid-session flight-mode reactivity) but neither PR baked the
docs. Add the user story to FUNCTIONAL.md (+ _ru mirror) and the mechanism to the
ARCHITECTURE offline section: the auto vs deliberate distinction, the cold-start
navigator.onLine / bounded reachability-probe decision, the mid-session window
online/offline events backed by a navigator.onLine poll (the events are unreliable
on some platforms, notably iOS PWAs), and the reachability probe being the one call
exempt from the offline kill switch.