Add metric checkboxes to the daily counts chart

The Daily counts legend is now a Pees/Poos/Meals checkbox group, so you
can hide metrics and focus on just the ones you care about. The choice is
persisted per device, at least one metric is always kept visible, and the
y-axis and bar widths adapt to the selected metrics.

src/style.css

@@ -685,6 +685,15 @@ dialog menu {
 .lg.poo .sw { background: var(--poo); }
 .lg.eat .sw { background: var(--eat); }
 
+/* Interactive legend: each item is a checkbox that toggles its metric. */
+.legend-toggle label.lg { cursor: pointer; user-select: none; }
+.legend-toggle input[type="checkbox"] { margin: 0; cursor: pointer; }
+.legend-toggle label.pee input { accent-color: var(--pee); }
+.legend-toggle label.poo input { accent-color: var(--poo); }
+.legend-toggle label.eat input { accent-color: var(--eat); }
+/* Dim an unchecked item so it's clear its bars are hidden. */
+.legend-toggle label.lg:has(input:not(:checked)) { opacity: 0.5; }
+
 /* ---------- auth (login / register) ---------- */
 .auth-screen {
   position: fixed;