FileUpload

A real file input, visually hidden but reachable, with drag and drop on top.

Bits has no primitive for this, but Askr's server side does: the multipart
parser copies nothing, and `StoreIn` saves under a random name rather than the
client's. That is why this was cheaper than it looks.

It is built on a real `<input type="file">`, visually hidden but focusable —
`hidden` would make it unreachable by keyboard. The label *is* the button, so
clicking and Enter both open the picker without us writing either.

**Drag and drop is an addition, never the only way in.** Someone who cannot
drag has to get just as far.

`maxSize` rejects oversized files with a message that names them, and never
silently.