feat(home): htmx lazy-load cover images

Replace the eager <img> on the home page with an htmx placeholder <figure>
that fetches a /cover-fragment endpoint when it scrolls into view
(hx-trigger="revealed"), so heavy cover bytes load only on demand.
Add spinner + settle-fade transition CSS, and load htmx.min.js on home.

justfile

@@ -176,6 +176,16 @@ deploy-migrate:
     ssh xamxam "cd /var/www/xamxam && REPO_ROOT=/var/www/xamxam bash /tmp/migrate.sh"
     ssh xamxam "rm -f /tmp/migrate.sh"
 
+[group('deploy')]
+deploy-cover-webp dry_run='':
+    # Backfill small WebP cover thumbnails on the server (idempotent).
+    # Dry-run by default; pass --no-dry-run to actually generate. Runs as the
+    # SSH user (member of the xamxam group, dirs are setgid group-writable) so
+    # no sudo is needed. Deploy as www-data if group-write ever changes.
+    rsync -v scripts/generate-cover-webp.php xamxam:/tmp/generate-cover-webp.php
+    ssh xamxam "cd /var/www/xamxam && php /tmp/generate-cover-webp.php {{dry_run}}"
+    ssh xamxam "rm -f /tmp/generate-cover-webp.php"
+
 [group('deploy')]
 deploy-env:
     #!/usr/bin/env bash