feat(account): unlink provider + change-email edges (backend + gateway)

Unlink: POST /user/link/unlink (telegram|vk) via account.RemoveIdentity, refusing
the last identity; email is never unlinked. New fbs LinkUnlinkRequest + gateway
link.unlink op, returning the refreshed profile.

Change-email: purposeChange confirm-codes (RequestChangeCode/ConfirmChange) that
atomically replace the account's confirmed email (account.replaceEmailIdentity);
a new address owned by another account is refused without disclosure, never merged.
The one-tap deeplink handles purposeChange too. Reuses the LinkEmail* fbs tables;
gateway link.email.change.{request,confirm} ops + backendclient methods; branded
ru/en change-email copy.

pkg/fbs/scrabble.fbs

@@ -501,6 +501,12 @@ table LinkTelegramRequest {
   data:string;
 }
 
+// LinkUnlinkRequest detaches a platform identity (kind = "telegram" | "vk") from the
+// caller's account; email is never unlinked (it is changed).
+table LinkUnlinkRequest {
+  kind:string;
+}
+
 // LinkResult is the unified result of a confirm or merge step. status is "linked"
 // (bound to the caller), "merge_required" (the identity belongs to another account —
 // the secondary_* fields summarise it for the irreversible confirmation), or