feat: add guestbook with localStorage persistence

Visitors can leave a short message (up to 140 chars) via a panel
toggled from the bottom-left corner. Entries are stored in
localStorage (capped at 20), displayed newest-first, and
HTML-escaped before rendering.

Fixes #135

app.js

@@ -209,6 +209,74 @@ document.getElementById('debug-toggle').addEventListener('click', () => {
   }
 });
 
+// === GUESTBOOK ===
+const GUESTBOOK_KEY = 'nexus_guestbook';
+const GUESTBOOK_MAX = 20;
+
+/**
+ * Load guestbook entries from localStorage.
+ * @returns {Array<{name: string, msg: string, ts: number}>}
+ */
+function guestbookLoad() {
+  try {
+    return JSON.parse(localStorage.getItem(GUESTBOOK_KEY) || '[]');
+  } catch (_) {
+    return [];
+  }
+}
+
+/**
+ * Save an entry to localStorage (max GUESTBOOK_MAX entries, newest first).
+ * @param {string} name
+ * @param {string} msg
+ */
+function guestbookSave(name, msg) {
+  const entries = guestbookLoad();
+  entries.unshift({ name: name.trim().slice(0, 32), msg: msg.trim().slice(0, 140), ts: Date.now() });
+  localStorage.setItem(GUESTBOOK_KEY, JSON.stringify(entries.slice(0, GUESTBOOK_MAX)));
+}
+
+/**
+ * Render guestbook entries into the messages container.
+ */
+function guestbookRender() {
+  const container = document.getElementById('guestbook-messages');
+  const entries = guestbookLoad();
+  if (entries.length === 0) {
+    container.innerHTML = '<div class="guestbook-empty">No messages yet. Be the first!</div>';
+    return;
+  }
+  container.innerHTML = entries.map(e => {
+    const safeName = e.name ? e.name.replace(/&/g, '&amp;').replace(/</g, '&lt;') : 'Anonymous';
+    const safeMsg = e.msg.replace(/&/g, '&amp;').replace(/</g, '&lt;');
+    return `<div class="guestbook-entry"><div class="guestbook-entry-name">${safeName}</div><div class="guestbook-entry-msg">${safeMsg}</div></div>`;
+  }).join('');
+}
+
+const guestbookPanel = document.getElementById('guestbook-panel');
+const guestbookBtn = document.getElementById('guestbook-btn');
+const guestbookClose = document.getElementById('guestbook-close');
+const guestbookForm = document.getElementById('guestbook-form');
+
+guestbookBtn.addEventListener('click', () => {
+  guestbookRender();
+  guestbookPanel.classList.toggle('open');
+});
+
+guestbookClose.addEventListener('click', () => {
+  guestbookPanel.classList.remove('open');
+});
+
+guestbookForm.addEventListener('submit', (e) => {
+  e.preventDefault();
+  const name = /** @type {HTMLInputElement} */ (document.getElementById('guestbook-name')).value;
+  const msg = /** @type {HTMLTextAreaElement} */ (document.getElementById('guestbook-msg')).value.trim();
+  if (!msg) return;
+  guestbookSave(name, msg);
+  guestbookForm.reset();
+  guestbookRender();
+});
+
 // === WEBSOCKET CLIENT ===
 import { wsClient } from './ws-client.js';
 

index.html

@@ -41,6 +41,26 @@
     <span class="overview-hint">[Tab] to exit</span>
   </div>
 
+  <!-- Guestbook -->
+  <div id="guestbook-toggle" class="hud-controls" style="position: absolute; bottom: 16px; left: 16px;">
+    <button id="guestbook-btn" class="chat-toggle-btn" aria-label="Open guestbook" style="background-color: var(--color-secondary); color: var(--color-text); padding: 4px 10px; border-radius: 4px; font-size: 12px; cursor: pointer; border: 1px solid var(--color-primary);">
+      Guestbook
+    </button>
+  </div>
+
+  <div id="guestbook-panel" role="dialog" aria-modal="true" aria-label="Guestbook">
+    <div id="guestbook-header">
+      <span>&#10022; Guestbook</span>
+      <button id="guestbook-close" aria-label="Close guestbook">&#10005;</button>
+    </div>
+    <div id="guestbook-messages"></div>
+    <form id="guestbook-form" autocomplete="off">
+      <input id="guestbook-name" type="text" maxlength="32" placeholder="Your name (optional)" aria-label="Your name" />
+      <textarea id="guestbook-msg" maxlength="140" placeholder="Leave a message&#x2026;" required aria-label="Your message"></textarea>
+      <button type="submit">Sign</button>
+    </form>
+  </div>
+
   <script type="module" src="app.js"></script>
 </body>
 </html>

style.css

@@ -106,3 +106,136 @@ canvas {
   0%, 100% { opacity: 0.7; }
   50% { opacity: 1; }
 }
+
+/* === GUESTBOOK === */
+#guestbook-panel {
+  display: none;
+  position: fixed;
+  bottom: 56px;
+  left: 16px;
+  width: 280px;
+  max-height: 400px;
+  background: rgba(0, 0, 16, 0.88);
+  border: 1px solid var(--color-primary);
+  border-radius: 6px;
+  z-index: 30;
+  font-family: var(--font-body);
+  font-size: 12px;
+  color: var(--color-text);
+  box-shadow: 0 0 18px rgba(68, 136, 255, 0.18);
+  flex-direction: column;
+}
+
+#guestbook-panel.open {
+  display: flex;
+}
+
+#guestbook-header {
+  display: flex;
+  justify-content: space-between;
+  align-items: center;
+  padding: 8px 12px;
+  border-bottom: 1px solid var(--color-secondary);
+  letter-spacing: 0.15em;
+  color: var(--color-primary);
+  font-size: 11px;
+  text-transform: uppercase;
+}
+
+#guestbook-close {
+  background: none;
+  border: none;
+  color: var(--color-text-muted);
+  cursor: pointer;
+  font-size: 12px;
+  padding: 0 2px;
+  line-height: 1;
+}
+
+#guestbook-close:hover {
+  color: var(--color-text);
+}
+
+#guestbook-messages {
+  overflow-y: auto;
+  flex: 1;
+  padding: 8px 12px;
+  min-height: 60px;
+  max-height: 200px;
+}
+
+.guestbook-entry {
+  padding: 5px 0;
+  border-bottom: 1px solid rgba(51, 68, 136, 0.3);
+}
+
+.guestbook-entry:last-child {
+  border-bottom: none;
+}
+
+.guestbook-entry-name {
+  color: var(--color-primary);
+  font-size: 11px;
+  margin-bottom: 2px;
+}
+
+.guestbook-entry-msg {
+  color: var(--color-text);
+  font-size: 12px;
+  word-break: break-word;
+}
+
+.guestbook-empty {
+  color: var(--color-text-muted);
+  font-size: 11px;
+  text-align: center;
+  padding: 12px 0;
+}
+
+#guestbook-form {
+  display: flex;
+  flex-direction: column;
+  gap: 6px;
+  padding: 8px 12px;
+  border-top: 1px solid var(--color-secondary);
+}
+
+#guestbook-name,
+#guestbook-msg {
+  background: rgba(0, 0, 16, 0.6);
+  border: 1px solid var(--color-secondary);
+  border-radius: 3px;
+  color: var(--color-text);
+  font-family: var(--font-body);
+  font-size: 12px;
+  padding: 5px 8px;
+  outline: none;
+  resize: none;
+}
+
+#guestbook-name:focus,
+#guestbook-msg:focus {
+  border-color: var(--color-primary);
+}
+
+#guestbook-msg {
+  height: 56px;
+}
+
+#guestbook-form button[type="submit"] {
+  background-color: var(--color-primary);
+  color: var(--color-bg);
+  border: none;
+  border-radius: 3px;
+  padding: 5px 0;
+  font-family: var(--font-body);
+  font-size: 12px;
+  cursor: pointer;
+  letter-spacing: 0.1em;
+  transition: background-color 0.2s;
+}
+
+#guestbook-form button[type="submit"]:hover {
+  background-color: var(--color-secondary);
+  color: var(--color-text);
+}