fix(vk): persist settings via VK Bridge storage; hide competing sign-in in Mini App hosts

VK moderation follow-ups.

1. Settings (theme, language, board style, reduce-motion, zoom, first-run
   coachmark flags) were lost between reloads in VK's desktop iframe: Firefox
   partitions/blocks IndexedDB and localStorage in a cross-origin iframe, so the
   browser-local store is empty on every reload (a plain tab / PWA keeps them —
   first-party storage — and Telegram has its own CloudStorage sync). Mirror the
   prefs (plus the locale, which has no other durable client home on VK) and the
   coachmark flags to VK Bridge storage (VKWebAppStorageSet/Get), which travels
   over postMessage to vk.com and survives; reconcile them on the VK launch
   before the first paint. The VK identity re-derives from the signed launch
   params each load, so the values reload onto the same account.

2. Inside a proprietary Mini App host (VK or Telegram) the profile now surfaces
   only email linking — both the Telegram and VK link/unlink entries are hidden
   (signInProvidersVisible), so a competing sign-in is never advertised (a
   platform ToS requirement; VK forbids showing a Telegram login, mirrored in
   Telegram for VK). A provider linked earlier on the web stays attached and is
   managed from the web; web/native builds show both.

Tests: vkprefs codec + signInProvidersVisible unit tests. Docs: ARCHITECTURE,
FUNCTIONAL (+_ru). No schema/wire change.

docs/FUNCTIONAL.md

@@ -129,9 +129,10 @@ to finish that shared game before signing in.
 
 The profile lists the account's **sign-in methods**. On the web a player can add
 Telegram (a login-widget popup) or VK (VK ID web login — a redirect to VK's sign-in and
-back); inside a Mini App the host platform is already linked, and its own sign-in-method
-row is hidden — the platform the player is currently signed in through cannot be unlinked
-from there, only the other provider's row is shown. Linking a provider that
+back); inside a proprietary Mini App host (VK or Telegram) the player is offered **only email
+linking** — both providers' rows and add buttons are hidden there, so a competing sign-in is
+never shown (a platform ToS rule). A provider linked earlier on the web stays attached and is
+managed from the web. Linking a provider that
 already belongs to another account offers the same irreversible **merge** as email
 linking. A linked provider can be **unlinked** — except the last
 remaining sign-in method, which is refused so the account stays reachable. **Email is
@@ -425,7 +426,9 @@ an email or Telegram and merging accounts are covered under "Accounts, linking &
 merge". Inside the Telegram Mini App, Telegram's own ⋮ menu also offers a **Settings**
 entry that opens this screen, and your display preferences (theme, board-label style and
 reduce-motion — not the interface language, which follows your account) sync across your
-Telegram devices. On a touch device the settings also offer a **Zoom the board** toggle (on by
+Telegram devices. Inside the VK Mini App the same preferences — and the interface language —
+are remembered between sessions this way too, including VK's desktop web version, where the
+browser would otherwise forget them on every reload. On a touch device the settings also offer a **Zoom the board** toggle (on by
 default): with it off, dropping a tile no longer auto-magnifies the board toward it. It is a
 per-device preference and is hidden on desktop, where the board already fits and never auto-zooms.