chore: add type:module, fix npm audit, rename CJS files, document module structure

- package.json: add type: module to silence ESM warnings
- server: npm audit fix (0 vulnerabilities, ws vuln resolved)
- Rename 4 CJS files to .cjs: build-extension, test-content-video-finder,
  test-locales, website/build
- Update eslint.config.mjs for .cjs glob patterns
- extension/README.md + server/README.md: document module structure

scripts/test-content-video-finder.cjs

@@ -0,0 +1,64 @@
+const fs = require('fs');
+const path = require('path');
+const assert = require('assert');
+
+const contentPath = path.join(__dirname, '..', 'extension', 'content.js');
+const source = fs.readFileSync(contentPath, 'utf8');
+
+function extractFunction(name, text) {
+  const start = text.indexOf(`function ${name}`);
+  assert.notStrictEqual(start, -1, `${name} not found`);
+
+  const bodyStart = text.indexOf('{', start);
+  let depth = 0;
+  for (let i = bodyStart; i < text.length; i++) {
+    if (text[i] === '{') depth++;
+    if (text[i] === '}') depth--;
+    if (depth === 0) return text.slice(start, i + 1);
+  }
+  throw new Error(`${name} body did not terminate`);
+}
+
+function makeVideo(name, width, height, options = {}) {
+  return {
+    name,
+    tagName: 'VIDEO',
+    videoWidth: width,
+    videoHeight: height,
+    offsetWidth: width,
+    offsetHeight: height,
+    muted: options.muted ?? true,
+    duration: options.duration ?? 0
+  };
+}
+
+const lightPreview = makeVideo('light-preview', 160, 90, { muted: false, duration: 30 });
+const shadowPlayer = makeVideo('shadow-player', 1920, 1080, { muted: false, duration: 3600 });
+
+const shadowRoot = {
+  querySelectorAll(selector) {
+    if (selector === 'video') return [shadowPlayer];
+    return [];
+  }
+};
+
+const shadowHost = { shadowRoot };
+
+const fakeDocument = {
+  querySelectorAll(selector) {
+    if (selector === 'video') return [lightPreview];
+    return [shadowHost];
+  }
+};
+
+const fnSource = extractFunction('findVideo', source);
+const findVideo = Function('document', `${fnSource}; return findVideo;`)(fakeDocument);
+
+const selected = findVideo(fakeDocument);
+assert.strictEqual(
+  selected,
+  shadowPlayer,
+  'findVideo should score Shadow DOM videos together with light DOM videos'
+);
+
+console.log('content video finder tests passed');