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>

backend/src/server.ts

@@ -17,6 +17,7 @@ import {
   removeMember,
   seedDefaultPods,
 } from './pods/store.js';
+import { getPresence } from './livekit/rooms.js';
 import type { InterventionOutcome } from '@podman/shared';
 
 const app = express();
@@ -72,6 +73,15 @@ app.get('/api/memory/stats', async (_req, res) => {
   }
 });
 
+// Live presence: who is currently connected in each pod's LiveKit room.
+app.get('/api/presence', async (_req, res) => {
+  try {
+    res.json(await getPresence());
+  } catch (e) {
+    res.status(500).json({ error: (e as Error).message });
+  }
+});
+
 // --- Pods CRUD (Mongo-backed) ---
 app.get('/api/pods', async (_req, res) => {
   try {