feat: one-click join + live pod presence

Join UX: click a member's name to join directly, or type a name and "Join"
(adds to roster + joins) — removes the redundant dropdown step. "Add" still
adds to the roster without joining.

Presence: new GET /api/presence (LiveKit RoomServiceClient) reports who's
connected per pod. The pod list polls it and shows a "N in room" badge plus a
green dot on members currently connected, so people can see active pods and jump in.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

frontend/src/lib/api.ts

@@ -41,6 +41,11 @@ export async function listPods(): Promise<Pod[]> {
   return json(await fetch(`${BACKEND_URL}/api/pods`));
 }
 
+/** Display names currently connected per pod id (= LiveKit room name). */
+export async function getPresence(): Promise<Record<string, string[]>> {
+  return json(await fetch(`${BACKEND_URL}/api/presence`));
+}
+
 export async function createPod(input: PodInput): Promise<Pod> {
   return json(
     await fetch(`${BACKEND_URL}/api/pods`, {