Mark the session on screen as Current in the session lists

- The conversation page tells the rail and the sessions modal which
  session it shows (announceShownSession, useShownSession); they reload,
  so a session reopened by ?session_id= shows at once.
- Only that session is Current and greyed out (the active one without
  ?session_id=); the active row then leads back to it.
- The rail lists the session on screen even when it's older than its
  latest sessions.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/functions/frontend/announce-shown-session.ts

@@ -0,0 +1,18 @@
+import type { ShownSession } from "@/src/types";
+
+export const ShownSessionEvent = "cobble:session-shown";
+
+/**
+ * The last session a conversation page showed, for a rail list that
+ * mounts after the page told it.
+ */
+export let lastShownSession: ShownSession | null = null;
+
+/**
+ * Tells the rail which session the conversation page shows, once its
+ * history loads (`useShownSession`).
+ */
+export default function announceShownSession(shown: ShownSession) {
+    lastShownSession = shown;
+    window.dispatchEvent(new CustomEvent(ShownSessionEvent, { detail: shown }));
+}