Make info panels collapsible with remembered state

Each main section with a data-panel key can now be folded by clicking its
heading (rotating chevron, keyboard-accessible via role/tabindex/aria-expanded
and Enter/Space). Collapsed keys are persisted in localStorage and restored on
load, so the layout stays how you left it across reloads.

State is device-global like the theme (a single non-namespaced key, not
per-user data), and only collapsed panels are stored — so any panel added
later defaults to open.

src/style.css

@@ -756,3 +756,24 @@ input.switch:checked::after { transform: translateX(18px); }
 .chart-svg .hm-poo { fill: var(--poo); }
 .chart-svg .hm-eat { fill: var(--eat); }
 
+/* ---------- collapsible panels ---------- */
+section.collapsible > h2 {
+  cursor: pointer;
+  position: relative;
+  padding-right: 20px;
+  -webkit-user-select: none;
+  user-select: none;
+}
+section.collapsible > h2::after {
+  content: "▾";
+  position: absolute;
+  right: 0;
+  top: 50%;
+  transform: translateY(-50%);
+  transition: transform 0.15s ease;
+  color: var(--muted);
+  font-size: 0.85em;
+}
+section.collapsed > h2::after { transform: translateY(-50%) rotate(-90deg); }
+section.collapsed > h2 { margin-bottom: 0; }
+section.collapsed > :not(h2) { display: none; }