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.

backend/internal/server/handlers.go

@@ -74,6 +74,12 @@ func (s *Server) registerRoutes() {
 		u.POST("/link/email/merge", s.handleLinkEmailMerge)
 		u.POST("/link/telegram", s.handleLinkTelegram)
 		u.POST("/link/telegram/merge", s.handleLinkTelegramMerge)
+		u.POST("/link/unlink", s.handleUnlink)
+		// Change the account's confirmed email: mail a code to the new address, then
+		// atomically switch on confirm (a new address owned by another account is
+		// refused without disclosure, never merged).
+		u.POST("/link/email/change/request", s.handleChangeEmailRequest)
+		u.POST("/link/email/change/confirm", s.handleChangeEmailConfirm)
 	}
 	if s.games != nil {
 		u.GET("/games", s.handleListGames)
@@ -231,6 +237,8 @@ func statusForError(err error) (int, string) {
 		return http.StatusUnprocessableEntity, "illegal_play"
 	case errors.Is(err, account.ErrEmailTaken), errors.Is(err, account.ErrIdentityTaken):
 		return http.StatusConflict, "email_taken"
+	case errors.Is(err, account.ErrLastIdentity):
+		return http.StatusConflict, "last_identity"
 	case errors.Is(err, accountmerge.ErrActiveGameConflict):
 		return http.StatusConflict, "merge_active_game_conflict"
 	case errors.Is(err, account.ErrInvalidEmail):