feat(stats): add stats page and pile trend

Show the stat groups on /stats, the pile trend on the dashboard card,
one switch per group in the settings, and the shared groups on the
public profile.

src/hooks/useUserStats.js

@@ -0,0 +1,39 @@
+import {useCallback, useEffect, useState} from 'react';
+import {apiFetch} from '../utils/api.js';
+import {statsUrl} from '../utils/stats.js';
+
+/**
+ * GET /api/library/stats for the given groups (none = all). stats is null while loading and after an error.
+ *
+ * @param onUnauthorized called on a 401, e.g. useSessionExpired
+ */
+export default function useUserStats(groups, onUnauthorized, enabled = true) {
+    const [stats, setStats] = useState(null);
+    const [failed, setFailed] = useState(false);
+    const key = groups.join(',');
+
+    const load = useCallback(async () => {
+        setFailed(false);
+        try {
+            const response = await apiFetch(statsUrl(key));
+            if (response.status === 401) {
+                onUnauthorized();
+                return;
+            }
+            if (!response.ok) {
+                setFailed(true);
+                return;
+            }
+            setStats(await response.json());
+        } catch (err) {
+            console.error('Stats load error:', err);
+            setFailed(true);
+        }
+    }, [key, onUnauthorized]);
+
+    useEffect(() => {
+        if (enabled) load();
+    }, [enabled, load]);
+
+    return {stats, failed, reload: load};
+}