Navigating between message threads with browser Back/Forward leaves openingThreadId stale and blocks MessagesPanel #4780

After #4777 (c9521d1), MessagesLayout in both apps/web/app/(ee)/partners.dub.co/(dashboard)/messages/layout.tsx and apps/web/app/app.dub.co/(dashboard)/[slug]/(ee)/program/messages/layout.tsx tracks openingThreadId and loadedThreadId to hide the previous thread until the newly opened thread finishes loading:

useEffect(() => {
  if (!programSlug) {
    setOpeningThreadId(null);
    setLoadedThreadId(null);
  }
}, [programSlug]);

And in PartnerMessagesProgramPage / ProgramMessagesPartnerPageClient:

const showingOpenedThread = !openingThreadId || openingThreadId === programSlug;

However, openingThreadId is only updated when clicking a different item in MessagesList (if (group.id !== activeId) setOpeningThreadId(group.id)) or ProgramSelector / PartnerSelector, and the useEffect in layout.tsx only resets openingThreadId when !programSlug (!partnerId).

As a result:

1. If you open thread A (/messages/prog-a), click thread B (/messages/prog-b) in MessagesList (which sets openingThreadId = "prog-b" and loadedThreadId = "prog-b"), and then press the browser Back button to return to /messages/prog-a, openingThreadId stays "prog-b" while programSlug is "prog-a". showingOpenedThread evaluates to false, setLoadedThreadId("prog-a") never runs, MessagesPanel receives messages={undefined}, and showThreadSpinner (openingThreadId !== loadedThreadId) stays stuck if loadedThreadId ever differs.
2. Similarly, if you click thread B in MessagesList (setOpeningThreadId("prog-b")) and then click back on thread A before Next.js finishes changing activeId from "prog-a", group.id !== activeId is false so openingThreadId stays "prog-b" and the spinner overlay stays up indefinitely.

Syncing setOpeningThreadId(programSlug ?? null) (and setOpeningThreadId(partnerId ?? null)) in layout.tsx when the route param changes and setting setOpeningThreadId unconditionally on click in MessagesList / ProgramSelector / PartnerSelector fixes both cases.

Environment:

• OS: Linux (Ubuntu 24.04)
• Browser: Brave / Chromium 135
• Commit: f949c06