Add lightbox for all preview images (component, grid cell, PDF thumbs)

Click any thumbnail to open full-size in overlay. Click backdrop or
press Escape to close. PDF thumbs now clickable too.

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

public/index.html

@@ -25,6 +25,9 @@
 		</div>
 	</header>
 	<main id="main"></main>
+	<div id="lightbox" hidden>
+		<img id="lightbox-img" alt="">
+	</div>
 	<script type="module" src="/app.mjs"></script>
 </body>
 </html>