feat: add blog with archives, pagination, search, sharing and RSS

- Content collection (one folder per post) with zod schema
- /blog, category and tag archives with /sayfa/<n>/ pagination
- Post page: breadcrumbs, date badges, share bar, TOC, DIY notice,
  code copy, license line, prev/next, BlogPosting + BreadcrumbList JSON-LD
- Sidebar: Pagefind search, categories, featured fallback, tag cloud
- Sätteri HAST plugins for figure/figcaption and external links
- Dual-theme Shiki, themed prose, RSS feed, latest posts on home
- Three sample posts marked as examples

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

src/components/blog/BlogListing.astro

@@ -0,0 +1,71 @@
+---
+// Shared layout for /blog/, category and tag archives (all paginated).
+import BaseLayout from '../../layouts/BaseLayout.astro';
+import { blog, fmt } from '../../lib/data';
+import type { Post } from '../../lib/posts';
+import Container from '../Container.astro';
+import GridLines from '../GridLines.astro';
+import SectionHeading from '../SectionHeading.astro';
+import Pager from './Pager.astro';
+import PostCard from './PostCard.astro';
+import Sidebar from './Sidebar.astro';
+
+export interface Props {
+  eyebrow: string;
+  title: string;
+  description: string;
+  /** Posts on this page. */
+  posts: Post[];
+  /** All published posts, for the sidebar. */
+  allPosts: Post[];
+  current: number;
+  total: number;
+  base: string;
+  activeCategory?: string;
+  activeTag?: string;
+  jsonLd?: Record<string, unknown>[];
+}
+
+const {
+  eyebrow,
+  title,
+  description,
+  posts,
+  allPosts,
+  current,
+  total,
+  base,
+  activeCategory,
+  activeTag,
+  jsonLd,
+} = Astro.props;
+const seoTitle = current > 1 ? fmt(blog.list.pageTitle, { title, n: current }) : title;
+---
+
+<BaseLayout title={seoTitle} description={description} jsonLd={jsonLd}>
+  <section class="surface-alt relative overflow-hidden">
+    <GridLines />
+    <Container class="relative py-14 md:py-20">
+      <SectionHeading level={1} eyebrow={eyebrow} title={title} />
+      <p class="mt-4 max-w-2xl text-lg">{description}</p>
+    </Container>
+  </section>
+
+  <Container class="grid gap-12 py-14 lg:grid-cols-[minmax(0,1fr)_20rem]">
+    <div>
+      {posts.length ? (
+        <ul class="flex flex-col gap-8">
+          {posts.map((post, i) => (
+            <li>
+              <PostCard post={post} priority={i === 0} />
+            </li>
+          ))}
+        </ul>
+      ) : (
+        <p class="border-fg/30 border-2 border-dashed p-8 text-center text-lg">{blog.list.empty}</p>
+      )}
+      <Pager current={current} total={total} base={base} />
+    </div>
+    <Sidebar posts={allPosts} activeCategory={activeCategory} activeTag={activeTag} />
+  </Container>
+</BaseLayout>