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feat(background): walk the follow list in the worker, not in the popup master

It is one request per hundred follows, which makes it the only read here whose length is set by how sociable the user is rather than by what is on the page: an account following fifty thousand people is five hundred requests. That must not be something a window is waiting on. The popup now asks the worker for the set and draws whatever comes back whenever it comes back, and closing the popup no longer throws a walk in progress away. The worker holds one walk per account so two cards asking at once still cost one, and answers with the DID it read for — an account switch between ask and answer makes that different from the one the popup asked about, and the label is what the cards check before drawing. `send` moves out of popup.ts so a card can use it without importing the module that imports the cards.