PROPER multiple account support for the same ID

like right now it's really annoying if you want to switch between multiple accounts you have to switch between multiple masterseeds

technicality suggestion:
```js
"masterseed": {
"certs": {
"issuer.bit": {
"auth_address": "",
"auth_privatekey": "",
},
```
should turn the object of issuer into array then object
```js
"masterseed": {
"certs": {
"issuer.bit": [
{
"auth_address": "",
"auth_privatekey": "",
},
//...
],
```
so users can have multiple ids of the same issuer, useful for sites like milchan which is anonymous orientated

Why?
because for so long we been switching using multi user, for once maybe we should finally have a proper solution for this

No files, tests, or entry points are mentioned. Start by tracing how the masterseed certs object and issuer lookups are read and written, then map the account-switching flow. Done means multiple certificates for one issuer can coexist and users can switch between those accounts without changing masterseeds.

索引モデルが issue の本文から書いたものです。