feat: Add configurable auto logout on inactivity

Users can pick an inactivity timeout in their preferences. A warning
appears 30 seconds before, and activity or a logout is synchronised
across open tabs.

client/src/utils/auto-logout-channel.js

@@ -0,0 +1,90 @@
+/*!
+ * Copyright (c) 2024 PLANKA Software GmbH
+ * Licensed under the Fair Use License: https://github.com/plankanban/planka/blob/master/LICENSE.md
+ */
+
+const CHANNEL_NAME = 'planka-auto-logout';
+const STORAGE_KEY = 'planka:auto-logout-message';
+
+const TAB_ID = `${Date.now().toString(36)}-${Math.random().toString(36).slice(2, 10)}`;
+
+const supportsBroadcastChannel = typeof window !== 'undefined' && 'BroadcastChannel' in window;
+
+let channel = null;
+const listeners = new Set();
+
+const dispatch = (message) => {
+  listeners.forEach((listener) => {
+    try {
+      listener(message);
+    } catch {
+      /* empty */
+    }
+  });
+};
+
+const handleStorageEvent = (event) => {
+  if (event.key !== STORAGE_KEY || !event.newValue) {
+    return;
+  }
+  try {
+    const message = JSON.parse(event.newValue);
+    if (message && message.tabId !== TAB_ID) {
+      dispatch(message);
+    }
+  } catch {
+    /* empty */
+  }
+};
+
+const ensureChannel = () => {
+  if (channel || typeof window === 'undefined') {
+    return;
+  }
+
+  if (supportsBroadcastChannel) {
+    channel = new BroadcastChannel(CHANNEL_NAME);
+    channel.addEventListener('message', (event) => {
+      if (event.data && event.data.tabId !== TAB_ID) {
+        dispatch(event.data);
+      }
+    });
+  } else {
+    window.addEventListener('storage', handleStorageEvent);
+    channel = { fallback: true };
+  }
+};
+
+const post = (message) => {
+  ensureChannel();
+  if (!channel) {
+    return;
+  }
+
+  const payload = { ...message, tabId: TAB_ID };
+
+  if (channel.fallback) {
+    try {
+      window.localStorage.setItem(STORAGE_KEY, JSON.stringify({ ...payload, _ts: Date.now() }));
+      window.localStorage.removeItem(STORAGE_KEY);
+    } catch {
+      /* empty */
+    }
+  } else {
+    channel.postMessage(payload);
+  }
+};
+
+export const postActivity = () => post({ type: 'activity' });
+
+export const postLogout = () => post({ type: 'logout' });
+
+export const postStayLoggedIn = () => post({ type: 'stay-logged-in' });
+
+export const subscribe = (handler) => {
+  ensureChannel();
+  listeners.add(handler);
+  return () => {
+    listeners.delete(handler);
+  };
+};