Add PDF first-page thumbnails via pdftoppm

Generated at upload time, stored alongside the PDF in data/pdfs/.
Shown in the file picker (48px) and component detail view (80px).
Gracefully skipped if pdftoppm is unavailable.

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

public/style.css

@@ -1574,7 +1574,23 @@ nav {
 .detail-file-row {
 	display: flex;
 	align-items: center;
-	gap: 0.5rem;
+	gap: 0.75rem;
+}
+
+.pdf-thumb {
+	width: auto;
+	height: 80px;
+	border: 1px solid var(--border);
+	border-radius: 3px;
+	flex-shrink: 0;
+}
+
+.fp-thumb {
+	width: auto;
+	height: 48px;
+	border: 1px solid var(--border);
+	border-radius: 3px;
+	flex-shrink: 0;
 }
 
 .detail-file-link {