fix(ui): add defensive sorting for blog posts

Implement client-side sorting for blog post lists to ensure they are
displayed in descending order by numeric ID. This prevents issues
where API response ordering or file modification timestamps might
cause older posts to appear newer than recent ones.

Also add backend tests to verify blog ordering logic.

admin/dashboard.html

@@ -234,7 +234,13 @@
         s.textContent = 'Načítám…';
         const res = await fetch('/api/blog/latest?limit=12');
         if (!res.ok) throw new Error('HTTP '+res.status);
-        const items = await res.json();
+        let items = await res.json();
+        // Defensive sort: numeric ID descending ensures newest first regardless of API ordering
+        items.sort((a,b)=>{
+          const ai = parseInt(a.id,10); const bi = parseInt(b.id,10);
+          if (!isNaN(ai) && !isNaN(bi)) return bi-ai;
+          return (b.id||'').localeCompare(a.id||'');
+        });
         grid.innerHTML='';
         if (!Array.isArray(items) || items.length === 0) {
           grid.innerHTML = '<div class="muted">Žádné příspěvky.</div>';