In compact layout the add-to-do menu no longer takes a row of its
own — it now renders inline on the last visible to-do, saving one
line per group. A compact group is guaranteed to have at least one
visible to-do (CategoryLane hides it otherwise), so there's always
a row to attach it to; a defensive fallback keeps the "+" reachable
in the off chance that invariant ever changes. Non-compact layout
(popover, AI add, archive, progress pie) is unchanged.
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