feat: <prop-viewer> fullscreen prop overlay

Implements fullscreen image overlay component for displaying art to players.
Methods: show(src, label), dismiss(), isOpen(). Starts hidden. Clicking
overlay dismisses. Strict TDD: 3 tests passing.

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

src/components/prop-viewer.js

@@ -0,0 +1,22 @@
+export class PropViewer extends HTMLElement {
+  constructor() { super(); this._src = ''; this._label = ''; }
+  connectedCallback() { if (!this.hasAttribute('hidden')) this.setAttribute('hidden', ''); this.render(); }
+
+  isOpen() { return !this.hasAttribute('hidden'); }
+
+  show(src, label = '') {
+    this._src = src; this._label = label;
+    this.removeAttribute('hidden');
+    this.render();
+  }
+  dismiss() { this.setAttribute('hidden', ''); }
+
+  render() {
+    this.innerHTML = `
+      <div class="prop-overlay" data-role="overlay">
+        <img class="prop-img" data-role="prop-img" src="${this._src}" alt="${this._label}" />
+      </div>`;
+    this.querySelector('[data-role=overlay]').addEventListener('click', () => this.dismiss());
+  }
+}
+customElements.define('prop-viewer', PropViewer);