Enable full mode with multiple rooms

src/lib/active.svelte.ts

@@ -0,0 +1,17 @@
+import { GROUP_ID, MODE } from "$lib/config";
+
+// The group the user is currently acting within (posting, chatting, joining).
+// Simple mode has exactly one group; full mode follows the room or thread being
+// viewed. GROUP_ID is never assumed in full mode — this is the single source of
+// truth for "which group".
+let current = $state<string>(MODE === "full" ? "" : GROUP_ID);
+
+export const activeGroup = {
+  get id() {
+    return current;
+  },
+};
+
+export function setActiveGroup(id: string) {
+  if (id) current = id;
+}