docs: correct VK invite-link reality + note desktop board drag-to-pan

Two doc edits that did not land alongside their code:

- FUNCTIONAL (+_ru) claimed a friend invitation is shareable as a Telegram
  *or VK* deep link "that opens it directly". On VK that is false: vkShareLink
  emits a plain vk.com/app<id> link (VK forwards no payload to the Mini App),
  so the recipient enters the copied code by hand (#142, commit d76f1f4 pivoted
  off the earlier #f<code> plan; the doc kept the old wording).
- UI_DESIGN omitted the desktop / landscape-iframe mouse drag-to-pan of the
  zoomed board added in #141 (touch scrolls the viewport natively; a mouse
  cannot, so an explicit drag-to-pan handler moves it).

docs/FUNCTIONAL.md

@@ -125,7 +125,8 @@ and any incidental perpendicular words are ignored and not scored — while on i
 standard Scrabble. English games are always standard and show no such toggle. In auto-match
 the choice joins the pairing key, so a player only meets opponents who picked the same rule. Friend games (2–4) are
 formed by inviting players from the friend list (an invitation, like a friend code,
-is shareable as a Telegram or VK deep link that opens it directly): the inviter chooses the
+is shareable as a Telegram deep link that opens it directly — on VK, which forwards no payload to the
+Mini App, the link only opens the app and the recipient enters the copied code by hand): the inviter chooses the
 settings and the game starts once every invitee has accepted — any decline cancels it, and an unanswered invitation
 expires after seven days.
 

docs/FUNCTIONAL_ru.md

@@ -131,7 +131,8 @@ _Вход сейчас только через провайдера, поэто
 показывают. В авто-подборе выбор входит в ключ подбора, поэтому игрок сводится только с теми,
 кто выбрал то же правило. Игры с друзьями (2–4)
 формируются приглашением игроков из списка друзей (приглашение, как и код друга,
-можно отправить deep-link'ом в Telegram или VK, который откроет его сразу): инициатор
+можно отправить deep-link'ом в Telegram, который откроет его сразу — на VK, который не передаёт Mini App
+никакой нагрузки, ссылка лишь открывает приложение, а скопированный код получатель вводит вручную): инициатор
 выбирает настройки, и партия стартует, когда приняли все приглашённые — любой отказ отменяет приглашение, а без
 ответа приглашение протухает через семь дней.
 

docs/UI_DESIGN.md

@@ -137,7 +137,10 @@ dismisses as soon as the lobby is ready. The pure layout and timing live in `lib
   the first time. A **swipe down on the zoom-out board** opens the history, but only when the
   board is scrolled to its top so it never fights the stage's own vertical scroll (the conflict
   that once retired this gesture) — and it is suppressed on the zoomed board, where the
-  one-finger drag pans. History also opens on a **tap of the score bar** and closes on a tap or
+  one-finger drag pans (and on desktop / the landscape iframe, where a mouse cannot drag-scroll the
+  viewport natively, a **drag-to-pan** handler moves the zoomed board instead — active only while
+  zoomed, off pending tiles, past a small threshold, swallowing the trailing click). History also
+  opens on a **tap of the score bar** and closes on a tap or
   an **upward swipe** of the then-inert board (below). A **hint** auto-zooms centred on the
   hint's placement, not
   the top-left.