Notification bell: pause polling when tab hidden, backoff on failure (#59)

setInterval(loadNotifications, 60000) ran unconditionally regardless
of tab visibility, and failures retried at the same fixed 60s cadence
forever. Now skips polling while document.hidden, resumes immediately
via visibilitychange when the tab regains focus, and backs off
exponentially (capped at 5 min) on repeated fetch failures, resetting
to the normal 60s cadence on the next success.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01MGDKHiU5RJdo3dqQUDow3X

views/layout_footer.php

@@ -235,11 +235,12 @@
       }
 
       function loadNotifications() {
-        fetch('/api/notifications.php', { credentials: 'same-origin' })
+        return fetch('/api/notifications.php', { credentials: 'same-origin' })
           .then(function(r) { return r.json(); })
-          .then(renderNotifications)
+          .then(function(data) { renderNotifications(data); return true; })
           .catch(function() {
             list.innerHTML = '<div style="padding:0.75rem;font-size:0.75rem;color:var(--text-muted);text-align:center">Could not load</div>';
+            return false;
           });
       }
 
@@ -261,9 +262,34 @@
       document.addEventListener('click', function(e) { if (_open && wrapEl && !wrapEl.contains(e.target)) closePanel(); });
       document.addEventListener('keydown', function(e) { if (e.key === 'Escape' && _open) closePanel(); });
 
-      // Initial badge count + poll every 60s
+      // Poll every 60s while the tab is visible, backing off (up to 5 min) on
+      // repeated failures, and resuming immediately when the tab regains focus.
+      var POLL_INTERVAL = 60000;
+      var MAX_POLL_INTERVAL = 300000;
+      var _pollTimer = null;
+      var _failCount = 0;
+
+      function scheduleNextPoll(delay) {
+        clearTimeout(_pollTimer);
+        _pollTimer = setTimeout(pollNotifications, delay);
+      }
+
+      function pollNotifications() {
+        if (document.hidden) return;
+        loadNotifications().then(function(ok) {
+          _failCount = ok ? 0 : _failCount + 1;
+          var delay = ok ? POLL_INTERVAL : Math.min(POLL_INTERVAL * Math.pow(2, _failCount), MAX_POLL_INTERVAL);
+          scheduleNextPoll(delay);
+        });
+      }
+
+      document.addEventListener('visibilitychange', function() {
+        if (!document.hidden) pollNotifications();
+      });
+
+      // Initial badge count, then start the poll cycle
       loadNotifications();
-      setInterval(loadNotifications, 60000);
+      scheduleNextPoll(POLL_INTERVAL);
     })();
 <?php endif ?>