mvmOS 0.36.0-beta: premium for mvmOS itself, starting with Apps Hub credits

Until now a subscription only ever unlocked something in an app you installed from the Store. From this release mvmOS itself can have premium features too, delivered exactly the same way: enter your key in Settings and your installation downloads the premium part from mvmos.org; remove the key and it is deleted again. There is no switch to flip, because an installation without a key never receives the code in the first place. Apps Hub credits are the first of them. Every public user gets a balance that any installed app can charge against or add to — a cost per generation, a reward for finishing something, a refund when a job fails — with a full transaction history, spends that cannot double-charge on a retry, and an admin view for topping up or correcting a balance. The two sides of the system behave differently on purpose. In the desktop the credits button stays exactly where it was even without a subscription: it opens the subscription window instead of a balance, because the person looking at it is the one who can actually get a key. On an app's public page there is nothing at all — no balance, no history, no mention of premium — because the people there are your users, and offering them something only you can buy would be noise. Apps asking for credits are told plainly that the feature is not there, so they fall back to working for free instead of showing a lock nobody can open. Balances that already exist are untouched. They stay in the database while a subscription is absent, and come back as they were the moment a key is active.