fix(frontend): restore chat DOM structure and guard addMessage against null

.github/instructions/DONE.md

@@ -1155,3 +1155,9 @@ Completed the three Playout enhancement workstreams from `TODO.md`. Remaining po
 - Bumped `frontend/package.json` to `1.0.13` and README version badge to `1.0.13`
 - Committed remaining work + `chore: bump version to 1.0.13`, tagged `1.0.13` (annotated)
 - Pushed `main` + `1.0.13` to origin → CI/CD + `release.yml` builds semver images and creates the Gitea release
+
+## Session 2026-08-13 — Frontend Error Fix
+
+- Fixed `Uncaught TypeError: Cannot read properties of null (reading 'appendChild')` in the viewer bundle: `index.html` was missing the `#messages` chat container (and had a stray `</section>`), so `addMessage` crashed on WebSocket open/message.
+- Restored chat panel structure: added `<section id="messages" class="chat-messages">` and wrapped `#presence-list` in `<section id="presence-panel" class="presence-panel">`.
+- Made `addMessage` defensive (returns early when `#messages` is absent), matching the null-guards in `renderPresence`/`showTyping`/`hideTyping`.