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>

docs/CHANGELOG.md

@@ -4,6 +4,16 @@ All notable changes to the KoalaSync browser extension and relay server.
 
 ---
 
+## [v2.6.1] — 2026-07-15
+
+### Fixed
+- **Extension: Episode Lobby peer list** — Peer names in the lobby are now rendered as text instead of markup. A peer could previously put HTML in their username and have it rendered in everyone else's popup, which allowed loading remote images (leaking viewer IP addresses) and spoofing the readiness badges. Scripts were already blocked by the extension's content security policy.
+
+### Changed
+- **Release checks: AMO validation** — `npm run verify` now runs Mozilla's `addons-linter` against the built Firefox artifact with `--warnings-as-errors`, and ESLint enforces `no-unsanitized`, so upload-blocking issues surface locally instead of at submission time.
+
+---
+
 ## [v2.6.0] — 2026-07-15
 
 ### Added