fix(security): add noreferrer to all target=_blank links

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

index.html

@@ -844,7 +844,7 @@ function renderList() {
                 <div class="brief-list-title${isActive ? ' active' : ''}">${esc(a.title)}</div>
                 ${metaParts.length ? `<div class="brief-list-meta">${metaParts.join('<span style="opacity:.4">·</span>')}</div>` : ''}
             </div>
-            ${a.url ? `<a class="brief-list-link" href="${esc(a.url)}" target="_blank" rel="noopener" aria-label="記事を開く">
+            ${a.url ? `<a class="brief-list-link" href="${esc(a.url)}" target="_blank" rel="noopener noreferrer" aria-label="記事を開く">
                 <i data-lucide="external-link" style="width:16px;height:16px;stroke-width:1.75"></i>
             </a>` : ''}
         `;