fix(seo): add h1 to homepage feed section

The page previously jumped from masthead (a <a>, not a heading) directly
to <h2> inside EntryCard, leaving the page without a primary heading.
Add a visible h1 introducing the feed: 'Avisos ambientals d'Eivissa
i Formentera'.

src/pages/index.astro

@@ -18,6 +18,7 @@ const fuentes = [...new Set(avisos.map((a) => a.fuente))].sort();
   </section>
 
   <section class="feed">
+    <h1 class="feed__title">Avisos ambientals d'Eivissa i Formentera</h1>
     <Filters municipios={municipios} temas={temas} fuentes={fuentes} />
 
     {avisos.length === 0 && (
@@ -54,6 +55,14 @@ const fuentes = [...new Set(avisos.map((a) => a.fuente))].sort();
     padding: var(--space-3) var(--space-2) var(--space-4);
   }
 
+  .feed__title {
+    font-family: var(--font-display);
+    font-size: 1.5rem;
+    font-weight: 700;
+    margin: 0 0 var(--space-2);
+    color: var(--ink);
+  }
+
   .empty {
     color: var(--ink-soft);
     padding: var(--space-2);