feat(variants): default a registered account to Erudit + Russian Scrabble

New Game opened on a single variant for everyone, which reads poorly on VK
where Russian Scrabble is the familiar game. A registered account now starts
with both Russian-alphabet games, so the picker offers a real choice with no
pre-selection.

A guest stays on Erudit alone and is invited to register for the rest: the
device-local guest has no profile at all, so the client-side fallback has to
keep matching the server. Registering promotes the set, but only while the
guest still carries the untouched guest default. Existing accounts are not
backfilled — the set they carry may be a deliberate choice.

The Telegram promo start-param becomes additive rather than a replacement,
with English Scrabble withheld from a Russian-speaking arrival; otherwise the
English campaign link would have taken Russian Scrabble away from every
account it onboarded.

docs/FUNCTIONAL.md

@@ -174,7 +174,8 @@ resigning or by letting it lapse to the 7-day timeout — drops from your *finis
 automatically, with no swipe needed; a normally finished AI game stays until you remove it, and
 no other game type is auto-removed. The game types offered on **New Game** are
 limited to the player's chosen **variant preferences** (see *Profile & settings*) —
-Erudite-first, defaulting to Erudite only. Variants are shown by their **display name** — both Scrabble variants read
+Erudite-first, defaulting to Erudite + Russian Scrabble for a registered player and to Erudite
+alone for a guest. Variants are shown by their **display name** — both Scrabble variants read
 "Scrabble"/"Скрэббл" and Erudit reads "Erudite"/"Эрудит" (by the interface language), and
 the same name titles the in-game screen. This gates only **starting** a new game you initiate —
 auto-match, a vs-AI game and a friend invitation **you create** — so a player still sees and plays
@@ -451,8 +452,11 @@ per-device preference and is hidden on desktop, where the board already fits and
 
 **Preferences (which variants you can be matched into).** A profile setting picks the game
 variants — Erudite, Russian Scrabble and English Scrabble, shown **Erudite-first** — you allow
-yourself to be matched into; a **new account starts with Erudite only** — unless it was created
-through the **promo bot's deep link**, which also enables **English Scrabble** — and you must keep
+yourself to be matched into; a **newly registered account starts with Erudite + Russian
+Scrabble**, and arriving through the **promo bot's deep link** also enables **English Scrabble**
+when your Telegram language is not Russian. A **guest** starts with **Erudite only** and New Game
+invites them to register for the rest; registering widens the account to the registered default.
+Changing the defaults never rewrites accounts that already exist. You must keep
 **at least one** selected. This list is exactly what **New Game** offers when you start a game
 (auto-match, an AI game, or a friend invitation you create) — a variant you have not enabled is
 not offered, and the server refuses it. It does not restrict games you are **invited** to: an