Post Platforms

"My platforms", all of them at once: {platforms: [...]}, any of gcg.platforms.FAMILIES' four, the empty list for every platform. The account as it now is, as POST /username answers, so the page that saved it hands it to lib/session.ts and the listings follow at once.

Kept each once and in FAMILIES' order, however it was sent. 422 invalid-platform for anything but a list of the four's exact names, and nothing is saved: a client that sent "ps5" meant a choice, and saving the rest of the list would answer it with a different one.

No password and no code: this changes what the account is shown, not how it is reached. The most a stolen cookie does with it is make the owner's choice for them, which the owner sees, and undoes, on the account page.