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/app.js

@@ -1769,6 +1769,48 @@
     if (ev) openEditDialog(ev);
   });
 
+  // ---------- collapsible panels ----------
+  // Each main section carrying a data-panel key can be folded by clicking its
+  // heading; collapsed keys are remembered across reloads. State is device-global
+  // (like the theme), so a single non-namespaced key is fine — it's not per-user
+  // data. Only collapsed panels are stored, so newly added panels default open.
+  const PANELS_KEY = "puppy-tracker:panels:v1";
+  function loadPanelState() {
+    try {
+      const s = JSON.parse(localStorage.getItem(PANELS_KEY));
+      return s && typeof s === "object" ? s : {};
+    } catch { return {}; }
+  }
+  function savePanelState(state) {
+    try { localStorage.setItem(PANELS_KEY, JSON.stringify(state)); } catch { /* ignore */ }
+  }
+  function initPanels() {
+    const state = loadPanelState();
+    document.querySelectorAll("main section[data-panel]").forEach(section => {
+      const key = section.dataset.panel;
+      const h2 = section.querySelector(":scope > h2");
+      if (!h2) return;
+      section.classList.add("collapsible");
+      const collapsed = !!state[key];
+      section.classList.toggle("collapsed", collapsed);
+      h2.setAttribute("role", "button");
+      h2.setAttribute("tabindex", "0");
+      h2.setAttribute("aria-expanded", String(!collapsed));
+      const toggle = () => {
+        const nowCollapsed = section.classList.toggle("collapsed");
+        h2.setAttribute("aria-expanded", String(!nowCollapsed));
+        const s = loadPanelState();
+        if (nowCollapsed) s[key] = true; else delete s[key];
+        savePanelState(s);
+      };
+      h2.addEventListener("click", toggle);
+      h2.addEventListener("keydown", (e) => {
+        if (e.key === "Enter" || e.key === " ") { e.preventDefault(); toggle(); }
+      });
+    });
+  }
+  initPanels();
+
   // ---------- wiring ----------
   document.querySelectorAll("button.action").forEach(btn => {
     btn.addEventListener("click", () => {