Subscriptions now count work in work units

Each subscription now says how much work it holds in work units, instead of dollars.

• Usage. Settings > Usage shows the 5-hour, week and month windows, each with its percent and when it frees up.
• A full window. Work waits and says when it opens. When the month is full, add more work for the month, move to a bigger subscription or use your own AI key.
• Extra work. Add half a month of work, up to twice a month. It lasts 12 months.
• Change size. Moving up keeps this month's work and charges only the difference. Moving down starts when the month ends.
• Free holds one worker and 5 work units a month, even with your own AI. A Free team with no work for 30 days pauses, with an email a week before. Nothing is deleted, and opening Little Worker starts it again.
• Only owners and admins can cancel or change the card. People in the EU and the UK can leave their email to hear when payments open.