fix: add null guard to _updateNotificationUI and .catch() to requestPermission

- _updateNotificationUI: add guard 'if (!statusEl || !reqBtn) return;' so
  future call sites without explicit null checks cannot throw TypeError
- Notification.requestPermission(): add .catch(console.error) for defensive
  error handling — promise always resolves in practice but unhandled rejections
  would be silently swallowed
- Tests: add test_update_notification_ui_has_null_guard and
  test_bind_static_event_listeners_permission_btn_has_catch to enforce both

435 tests pass (2 new)

Co-authored-by: Amplifier <amplifier@amp.dev>

muxplex/frontend/app.js

@@ -1026,6 +1026,7 @@ function onDisplaySettingChange() {
  * @param {string}  permission - Notification.permission value, or 'unsupported'.
  */
 function _updateNotificationUI(statusEl, reqBtn, permission) {
+  if (!statusEl || !reqBtn) return;
   if (permission === 'granted') {
     statusEl.textContent = 'Granted';
     reqBtn.disabled = true;
@@ -1385,6 +1386,8 @@ function bindStaticEventListeners() {
       if (statusEl && reqBtn) {
         _updateNotificationUI(statusEl, reqBtn, permission);
       }
+    }).catch(function(err) {
+      console.error('Notification.requestPermission() failed:', err);
     });
   });
 }

muxplex/tests/test_frontend_js.py

@@ -1404,3 +1404,31 @@ def test_bind_static_event_listeners_permission_btn_calls_request_permission() -
     assert "requestPermission" in body, (
         "bindStaticEventListeners permission button must call Notification.requestPermission()"
     )
+
+
+def test_bind_static_event_listeners_permission_btn_has_catch() -> None:
+    """Notification.requestPermission() in permission button handler must have a .catch() handler."""
+    match = re.search(
+        r"function bindStaticEventListeners\s*\(\s*\)\s*\{(.*?)\n\}",
+        _JS,
+        re.DOTALL,
+    )
+    assert match, "bindStaticEventListeners function not found"
+    body = match.group(1)
+    assert ".catch(" in body, (
+        "Notification.requestPermission() must have a .catch() handler for defensive error handling"
+    )
+
+
+def test_update_notification_ui_has_null_guard() -> None:
+    """_updateNotificationUI must guard against null inputs."""
+    match = re.search(
+        r"function _updateNotificationUI\s*\(.*?\)\s*\{(.*?)(?=\n\})",
+        _JS,
+        re.DOTALL,
+    )
+    assert match, "_updateNotificationUI function not found"
+    body = match.group(1)
+    assert "null" in body or "non-null" in body or "!statusEl" in body or "!reqBtn" in body, (
+        "_updateNotificationUI must include a null guard (null check) or JSDoc non-null annotation"
+    )