Moonchess: vanishing tic-tac-toe, online and practice modes

Online play over WebSockets with any-string game codes, seats and spectators,
and a 1-second heartbeat. Practice mode against Columbina, backed by a full
solver, with an accuracy slider, spoilers, undo and piece freshness.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

static/practice.html

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+<!doctype html>
+<html lang="en">
+<head>
+  <meta charset="utf-8">
+  <meta name="viewport" content="width=device-width, initial-scale=1">
+  <title>Moonchess Practice</title>
+  <link rel="stylesheet" href="/static/style.css">
+</head>
+<body>
+  <main>
+    <header class="bar">
+      <h1 class="page-title">Moonchess practice</h1>
+    </header>
+
+    <div class="players" role="group" aria-label="Choose your side">
+      <button class="player pickable" id="player-0"><span class="mark x">X</span><span class="who"></span></button>
+      <button class="player pickable" id="player-1"><span class="mark o">O</span><span class="who"></span></button>
+    </div>
+    <p class="hint">Click a side to play as it, any time. X moves first.</p>
+
+    <p id="status" class="status" aria-live="polite"></p>
+
+    <div id="board" class="board" role="grid" aria-label="Board"></div>
+
+    <div class="difficulty">
+      <label for="accuracy">Columbina's accuracy <output id="accuracy-value" for="accuracy"></output></label>
+      <input id="accuracy" type="range" min="0" max="100" step="5" value="50">
+      <p class="hint">Chance that each of Columbina's moves is perfect. Otherwise she plays a random empty square.</p>
+    </div>
+
+    <details class="spoiler">
+      <summary>Spoilers</summary>
+      <p id="spoiler-text"></p>
+      <label class="toggle"><input type="checkbox" id="freshness"> Show piece freshness</label>
+    </details>
+
+    <div class="footer">
+      <div class="actions">
+        <button id="undo" title="Undo (Ctrl+Z / Cmd+Z)">Undo</button>
+        <button id="new-game" class="primary">New game</button>
+      </div>
+      <p id="meta" class="meta"></p>
+    </div>
+  </main>
+  <script src="/static/board.js"></script>
+  <script src="/static/practice.js"></script>
+</body>
+</html>