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/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.