docs(offline): document connectivity auto-detect + transport kill switch

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.

docs/FUNCTIONAL.md

@@ -268,6 +268,15 @@ the app quietly preloads the dictionaries for the player's enabled variants whil
 (once installed) launches from a precached shell even with no network. The mode is device-scoped and
 sticky across launches.
 
+The app also **enters offline mode on its own** when it cannot reach the network. On a cold launch
+with no connection it switches to offline for that session; when the device is online but the gateway
+stays silent for a few seconds it asks first — *No connection. Switch to offline mode?*, with
+**Switch** (go offline) or **Wait for network** (keep retrying online). While the app is open, losing
+the network — airplane mode — switches to offline automatically, and restoring it returns online by
+itself. A self-entered offline is temporary: it reverts to online the moment the network is back, and
+the next launch re-checks. A **deliberate** offline — the Settings toggle, or **Switch** in that
+dialog — is the player's choice: it is kept, and never undone automatically.
+
 ### Social: friends, block, chat, nudge
 Become friends in two ways: redeem a **one-time code** the other player issues (six
 digits, valid for twelve hours), or send a **request to someone you have played