Check the playlist inbox when the app returns to the foreground and every two minutes while open

Sent playlists were only picked up on a cold start, so an installed PWA
resumed from the background never showed them.

frontend/app.js

@@ -562,7 +562,7 @@ function showSharedPlaylistModal(name, url, code) {
         msg.textContent = '\u26a0 ' + ((j && j.error) || 'Could not send');
       } else {
         msg.className = 'share-send-msg ok';
-        msg.textContent = `Sent to \u201c${to}\u201d \u2014 they'll see it next time they open YT Player.`;
+        msg.textContent = `Sent to \u201c${to}\u201d \u2014 they'll get it the next time YT Player is open on their device.`;
         input.value = '';
       }
     } catch {
@@ -727,10 +727,43 @@ async function adoptSharedPlaylistByCode(code, { fromName = '' } = {}) {
 // Playlists other users sent to this device's linked profile. Offered one at a
 // time on launch; each is cleared from the server inbox once answered, whether
 // the user kept it or not, so it is never offered twice.
+//
+// Launch alone is not enough: an installed PWA is usually resumed from the
+// background rather than relaunched, so it also re-checks when it returns to
+// the foreground and on a timer while visible (see startPlaylistInboxWatch).
+let inboxCheckRunning = false;
+let inboxLastCheck = 0;
 async function checkPlaylistInbox() {
-  if (!WEB) return;
+  if (!WEB || inboxCheckRunning) return;
   const me = data.profile && data.profile.name;
   if (!me) return; // deliveries are addressed to a profile, not a device
+  inboxCheckRunning = true;
+  inboxLastCheck = Date.now();
+  try {
+    await drainPlaylistInbox(me);
+  } finally {
+    inboxCheckRunning = false;
+  }
+}
+
+const INBOX_POLL_MS = 2 * 60 * 1000;
+const INBOX_MIN_GAP_MS = 20 * 1000;
+function startPlaylistInboxWatch() {
+  if (!WEB) return;
+  const maybeCheck = () => {
+    if (document.visibilityState !== 'visible') return;
+    if (Date.now() - inboxLastCheck < INBOX_MIN_GAP_MS) return;
+    // Never pop the prompt over a dialog the user is already in.
+    const modal = $('modal');
+    if (modal && !modal.classList.contains('hidden')) return;
+    checkPlaylistInbox();
+  };
+  document.addEventListener('visibilitychange', maybeCheck);
+  window.addEventListener('focus', maybeCheck);
+  setInterval(maybeCheck, INBOX_POLL_MS);
+}
+
+async function drainPlaylistInbox(me) {
 
   let items = [];
   try {
@@ -5356,6 +5389,7 @@ async function boot() {
   // After the profile is settled — a delivery is addressed to the profile, so
   // this must run once we know which one (if any) this device is linked to.
   await checkPlaylistInbox();
+  startPlaylistInboxWatch();
   applyAppearance();
   updateProfileStatus();
   updateLoopRepeatButtons();