feat: /blog pages point canonical at the root form

The root form is canonical (DESIGN.md): the alias keeps serving the same
page but declares alternates.canonical at me.nech.pl/<slug>, once per
post across the language variant too.

app/blog/[slug]/[lang]/page.js

 import { notFound } from "next/navigation";
 import PostArticle from "../../PostArticle";
 import { buildLanguageNav } from "../../language-nav";
-import { BLOG_LANGS, DEFAULT_LANG, getAllBlogSlugs, getBlogLanguages, getBlogPost } from "@/lib/blog";
+import { BLOG_LANGS, DEFAULT_LANG, blogCanonicalPath, getAllBlogSlugs, getBlogLanguages, getBlogPost } from "@/lib/blog";
 
 /**
  * Non-default language route: `/blog/<slug>/<lang>`. The default language is
@@ -25,14 +25,19 @@ export async function generateMetadata({ params }) {
   const post = await getBlogPost(slug, lang).catch(() => null);
   if (!post) return {};
 
+  // Alias with canonical: the canonical is declared once per post — the root
+  // form of the canonical folder slug (DESIGN.md, "the root form is
+  // canonical"). Language variants and root keys all point there.
+  const canonical = blogCanonicalPath(post.slug);
   return {
     title: post.title,
     description: post.description,
+    alternates: { canonical },
     openGraph: {
       title: post.title,
       description: post.description,
       type: 'article',
-      url: `/blog/${post.slug}`,
+      url: canonical,
       publishedTime: post.date,
       images: ['/images/profile.png'],
     },

app/blog/[slug]/page.js

 import { notFound } from "next/navigation";
 import PostArticle from "../PostArticle";
 import { buildLanguageNav } from "../language-nav";
-import { getAllBlogSlugs, getBlogPost } from "@/lib/blog";
+import { blogCanonicalPath, getAllBlogSlugs, getBlogPost } from "@/lib/blog";
 
 export function generateStaticParams() {
   const slugs = getAllBlogSlugs();
@@ -14,14 +14,18 @@ export function generateStaticParams() {
 export async function generateMetadata({ params }) {
   const { slug } = await params;
   const post = await getBlogPost(slug);
+  // Alias with canonical: the root form is the real address (DESIGN.md) —
+  // this page serves the same content and points at it.
+  const canonical = blogCanonicalPath(post.slug);
   return {
     title: post.title,
     description: post.description,
+    alternates: { canonical },
     openGraph: {
       title: post.title,
       description: post.description,
       type: 'article',
-      url: `/blog/${post.slug}`,
+      url: canonical,
       publishedTime: post.date,
       images: ['/images/profile.png'],
     },