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/lib/components/PostCard.svelte

@@ -97,7 +97,7 @@
         </span>
       </div>
     {/if}
-    {#if commentCount != null && commentCount > 0}
+    {#if commentCount != null}
       <div class="absolute top-2 left-2">
         <span
           class="inline-flex items-center gap-1 rounded-full bg-white/90 px-2 py-1 text-[.65rem] font-semibold tabular-nums text-zinc-700 shadow-sm"