Finding un-split songs took five taps (cycle each stem pill to its
"lacks" state) — and was quietly wrong even then: the drawer offered
five of the canonical six stems, so a piano-only song lacked all five
listed and matched a hand-built "not split" filter despite being split.
The stems section gains a "Not split" toggle that sets stem_lacks to
every instrument stem in one tap (the same lacks-ALL query Stem
Splitter's missing-stems view runs, backend semantics unchanged), and
piano joins the pill row (already in the backend's allowed set).
(Rebuilt on current main after #1003/#810/#92e78be rewrote the drawer
region — the original branch conflicted whole-file.)
Signed-off-by: topkoa <topkoa@gmail.com>
