shared playlists: a follow with news says so through a toast, and a follower's rows keep the owner's catalog ids

src/lib/state.svelte.js

@@ -256,6 +256,30 @@ export function announceProposal(from, playlist) {
     proposalToast.seq += 1;
 }
 
+/** A playlist this device follows has news (FollowNewsToast): the owner's
+ *  revision added rows, or dropped some. The follow poll runs every ten
+ *  minutes in silence and the Inbox card only shows when the page is open,
+ *  so without this a new track arrives unannounced. One poll can find news
+ *  on several follows at once: a second playlist inside the window counts
+ *  up ("3 playlists have new tracks") the way a burst of proposals does,
+ *  and the same playlist again just replaces its line. */
+export const followNewsToast = $state({ seq: 0, name: "", added: 0, removed: 0, count: 0 });
+const FOLLOW_NEWS_TOAST_WINDOW_MS = 8000;
+let followNewsToastAt = 0;
+let followNewsToastNames = new Set();
+
+export function announceFollowNews(name, added, removed) {
+    const now = Date.now();
+    if (now - followNewsToastAt >= FOLLOW_NEWS_TOAST_WINDOW_MS) followNewsToastNames = new Set();
+    followNewsToastNames.add(name);
+    followNewsToast.count = followNewsToastNames.size;
+    followNewsToast.name = name;
+    followNewsToast.added = added;
+    followNewsToast.removed = removed;
+    followNewsToastAt = now;
+    followNewsToast.seq += 1;
+}
+
 /** Background-import status, read by ImportToast. The import window keeps
  *  scanning/copying after it is closed (the component stays mounted, only
  *  its template goes), and closed must never mean invisible: while work
@@ -7809,6 +7833,7 @@ if (import.meta.env.DEV) {
         playlistStore, refreshPlaylists, goToView, peekNextIndices,
         slsk, search, runSearch, uploadToast, announceUpload,
         proposalToast, announceProposal,
+        followNewsToast, announceFollowNews,
         searchTabs, selectSearchTab, closeSearchTab, filesForGroup,
         downloads, network, sendDm, openChat,
     };