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>

tests/components/prop-viewer.test.js

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+import { describe, it, expect, beforeEach } from 'vitest';
+import '../../src/components/prop-viewer.js';
+
+describe('<prop-viewer>', () => {
+  let el;
+  beforeEach(() => {
+    document.body.innerHTML = '';
+    el = document.createElement('prop-viewer');
+    document.body.appendChild(el);
+  });
+
+  it('starts hidden', () => {
+    expect(el.isOpen()).toBe(false);
+    expect(el.hasAttribute('hidden')).toBe(true);
+  });
+
+  it('shows an image when show() is called', () => {
+    el.show('/art/rain-pier.png', 'Pier in the rain');
+    expect(el.isOpen()).toBe(true);
+    const img = el.querySelector('[data-role=prop-img]');
+    expect(img.getAttribute('src')).toBe('/art/rain-pier.png');
+  });
+
+  it('dismisses when the overlay is clicked', () => {
+    el.show('/art/x.png', 'x');
+    el.querySelector('[data-role=overlay]').click();
+    expect(el.isOpen()).toBe(false);
+  });
+});