Hide draft articles from the front page and mark drafts when viewed directly

Add an Article::published() scope and use it on the front page (which
previously showed drafts), the /blog listing and the feeds. Draft
articles are still reachable at their URL, but now show a draft notice,
a "[Draft]" title prefix and a noindex robots meta tag.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

resources/css/layout.css

@@ -63,3 +63,16 @@
   }
 }
 
+
+@layer components {
+  .draft-notice {
+    margin-block-end: 2ex;
+    padding: 1ex 1em;
+    border: 2px dashed var(--clr-border);
+    border-radius: 8px;
+    background: light-dark(
+      oklch(95% 0.08 85deg),
+      oklch(30% 0.08 85deg)
+    );
+  }
+}