fix(popup): seed lastKnownPeers in init() to prevent false join toasts

init() called updatePeerList() directly but never set lastKnownPeers.
On the first PEER_UPDATE message (heartbeat, play/pause, any state
change), detectPeerChanges() saw lastKnownPeers=[] and treated ALL
peers including self as newly joined — triggering 'name joined the room'
toast on every action while the popup was open.

extension/popup.js

@@ -109,6 +109,7 @@ async function init() {
             localPeerId = res.peerId;
             applyConnectionStatus(res.status);
             updatePeerList(res.peers);
+            lastKnownPeers = res.peers || [];
             if (res.lastActionState) updateLastActionUI(res.lastActionState, res.peers);
             
             // Populate Tabs using the background's targetTabId