feat: print-friendly layout + comment count on every post overview

- Hides Header / HeaderOverlay / Footer / TopBanner / Breadcrumbs and
  the comment accordion when printing. Action toolbar already had
  `print:hidden`.
- Adds @media print rules in app.css: 1.5cm page margins, 11pt body,
  forced black-on-white, expand link URLs after anchor text inside
  articles, keep images and headings together at page breaks.
- PostCard now renders the count pill whenever a value is known (incl.
  zero); previously hidden for `count === 0`, so posts whose comments
  are still pending always looked empty.
- Extracts the bulk-fetch helper to `lib/comment-counts.ts`
  (`fetchCommentCounts` + `postSlugForComments`) and uses it from both
  `BlogOverview` and `PostOverviewBlock`. Homepage / any page with a
  post_overview block now shows the badges too — previously only the
  paginated /posts/* routes did.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/app.css

@@ -506,3 +506,42 @@ footer a:hover,
     border-left: 1px solid var(--color-stein-200);
   }
 }
+
+/* ── Print rules ────────────────────────────────────────────────────────────
+   Goal: print only the article. `print:hidden` on chrome (header/footer/
+   banner/breadcrumbs/comments) handles structural removal. The rules below
+   set page margins, force black-on-white, expand links, and keep images
+   inside the page break. */
+@media print {
+  @page {
+    margin: 1.5cm;
+  }
+  html,
+  body {
+    background: white \!important;
+    color: black \!important;
+    font-size: 11pt;
+  }
+  /* Show the URL after each anchor inside the article so printed pages stay
+     useful when the screen-only links are gone. */
+  article a[href]:not([href^="#"])::after {
+    content: " (" attr(href) ")";
+    font-size: 0.85em;
+    color: #555;
+    word-break: break-all;
+  }
+  /* Don't break inside images, keep figures together with their captions. */
+  img,
+  figure {
+    break-inside: avoid;
+    max-width: 100% \!important;
+    height: auto \!important;
+  }
+  /* Keep headings with the next paragraph. */
+  h1,
+  h2,
+  h3,
+  h4 {
+    break-after: avoid;
+  }
+}