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/style.css

@@ -148,6 +148,31 @@ nav {
 	background: var(--surface-raised);
 }
 
+/* ===== LIGHTBOX ===== */
+
+#lightbox {
+	position: fixed;
+	inset: 0;
+	background: rgba(0,0,0,0.85);
+	display: flex;
+	align-items: center;
+	justify-content: center;
+	z-index: 1000;
+	cursor: zoom-out;
+}
+
+#lightbox[hidden] {
+	display: none;
+}
+
+#lightbox-img {
+	max-width: 90vw;
+	max-height: 90vh;
+	object-fit: contain;
+	border-radius: 3px;
+	box-shadow: 0 8px 40px rgba(0,0,0,0.6);
+}
+
 /* ===== MAIN ===== */
 
 #main {