feat(ui): add ContentPanel component to render readable HTML

- Shows loading/empty states
- Renders title and HTML via dangerouslySetInnerHTML
- Minimal API for integration

src/components/ContentPanel.tsx

@@ -0,0 +1,43 @@
+import React from 'react'
+
+interface ContentPanelProps {
+  loading: boolean
+  title?: string
+  html?: string
+  selectedUrl?: string
+}
+
+const ContentPanel: React.FC<ContentPanelProps> = ({ loading, title, html, selectedUrl }) => {
+  if (!selectedUrl) {
+    return (
+      <div className="content-panel empty">
+        <p>Select a bookmark to preview its content.</p>
+      </div>
+    )
+  }
+
+  if (loading) {
+    return (
+      <div className="content-panel loading">
+        <p>Loading content…</p>
+      </div>
+    )
+  }
+
+  return (
+    <div className="content-panel">
+      {title && <h2 className="content-title">{title}</h2>}
+      {html ? (
+        <div className="content-html" dangerouslySetInnerHTML={{ __html: html }} />
+      ) : (
+        <div className="content-panel empty">
+          <p>No readable content found for this URL.</p>
+        </div>
+      )}
+    </div>
+  )
+}
+
+export default ContentPanel
+
+