docs: sign-in methods matrix — unlink + change-email

FUNCTIONAL (+ru): drop the stale 'linking UI hidden' note; document the profile
sign-in-methods matrix, the unlink last-identity guard, and the non-disclosing
atomic email change. ARCHITECTURE: unlink + change-email behaviour and the
purpose=change deeplink branch.

docs/FUNCTIONAL.md

@@ -96,10 +96,6 @@ reconnect), and pending reads resume on their own — the interface stays usable
 flashing a red banner each time.
 
 ### Accounts, linking & merge
-_Sign-in is currently provider-only, so the in-profile linking UI is temporarily hidden; it
-returns once the anonymous `/app/` guest (whose upgrade path this is) ships. The flow below
-describes it for when it does._
-
 First platform contact auto-provisions a durable account. From the profile a player
 links an email (via a confirm code) or their Telegram (via the web sign-in); a guest
 who links their first identity becomes a durable account. The "already taken" status
@@ -111,6 +107,16 @@ when a guest links an identity that already has a durable account, where the dur
 account is kept and the guest's games move into it. A merge is blocked only while the
 two accounts share a game still in progress.
 
+The profile lists the account's **sign-in methods**. On the web a player can add
+Telegram; adding VK on the web is not offered yet, and inside a Mini App the host
+platform is already linked. A linked provider can be **unlinked** — except the last
+remaining sign-in method, which is refused so the account stays reachable. **Email is
+never unlinked; it is changed**: the player enters a new address, confirms a code
+mailed to it, and the account switches to it atomically, freeing the old address. A new
+address that already belongs to another account is refused with a neutral "check the
+address or contact support" — the switch never merges and never reveals the other
+account.
+
 ### Lobby & matchmaking
 On a cold open the lobby greets the player with a brief **loading splash** — Scrabble tiles
 spelling **ЭРУДИТ / ЗАГРУЗКА / ОЖИДАНИЕ** as a small crossword — that clears the moment the