fix(popup): render lobby peer names as text, not markup

Peer usernames are remote-controlled and were interpolated into an
innerHTML string in updateLobbyUI. The server only truncates them to 30
chars, so a peer could inject markup into everyone else's popup: enough
to load a remote image (leaking viewer IPs) or spoof readiness badges.
Inline handlers were already blocked by the MV3 default CSP.

Build the peer items with the DOM API, matching the pattern the sibling
peer list already uses.

Add the two checks that would have caught this before upload:
- eslint no-unsanitized, which reproduces the AMO warning at lint time
- addons-linter on the built XPI in verify-release, with
  --warnings-as-errors since it exits 0 on warnings and AMO rejects them

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

extension/popup.js

@@ -2603,9 +2603,10 @@ function updateLobbyUI(lobby, peers) {
     elements.episodeLobbyCard.style.display = 'block';
     elements.lobbyTitle.textContent = getMessage('LOBBY_WAITING_FOR', { title: lobby.expectedTitle });
 
-    // Build peer readiness list
+    // Build peer readiness list. Peer usernames are remote-controlled input, so
+    // every node is built with the DOM API and filled via textContent.
     const readySet = new Set(lobby.readyPeers || []);
-    const peerHtmls = [];
+    const peerItems = [];
 
     if (peers && peers.length > 0) {
         peers.forEach(p => {
@@ -2616,12 +2617,26 @@ function updateLobbyUI(lobby, peers) {
             const icon = isReady ? '✅' : '⏳';
             const label = isReady ? getMessage('LABEL_LOBBY_PEER_READY') : getMessage('LABEL_LOBBY_PEER_LOADING');
             const badgeClass = isReady ? 'badge-ready' : 'badge-loading';
-            peerHtmls.push(`<span class="lobby-peer-item">${icon} ${avatar} ${pName} <span class="badge ${badgeClass}">${label}</span></span>`);
+
+            const item = document.createElement('span');
+            item.className = 'lobby-peer-item';
+            item.appendChild(document.createTextNode(`${icon} ${avatar} ${pName} `));
+
+            const badge = document.createElement('span');
+            badge.className = `badge ${badgeClass}`;
+            badge.textContent = label;
+            item.appendChild(badge);
+
+            peerItems.push(item);
         });
     }
 
-    if (peerHtmls.length > 0 && elements.lobbyPeerStatus) {
-        elements.lobbyPeerStatus.innerHTML = peerHtmls.join(' ');
+    if (peerItems.length > 0 && elements.lobbyPeerStatus) {
+        elements.lobbyPeerStatus.textContent = '';
+        peerItems.forEach((item, index) => {
+            if (index > 0) elements.lobbyPeerStatus.appendChild(document.createTextNode(' '));
+            elements.lobbyPeerStatus.appendChild(item);
+        });
     } else if (elements.lobbyPeerStatus) {
         elements.lobbyPeerStatus.textContent = getMessage('LOBBY_WAITING_PEERS');
     }