The panel had an inset for this; the calendar never did. Its root filled the
leaf, and Obsidian's phone navbar floats over a leaf rather than shortening
it, so the scroll container ran to the bottom of the screen and the last row
of the list sat under the bar - unreachable.
`lib/inset.ts` is the same pure measurement the panel plugin grew, with the
same tests: the deeper of navbar and keyboard, never their sum, and a bar that
sits beside the view covers nothing of it. The view re-measures by frame until
it holds still and writes `--bcal-inset-bottom`.
`viewport()` subtracts the bar too, so anchored panels and the quick look are
placed against what is actually reachable instead of ending under it.
