Add numbered pagination and redesign the IT/EN language switch

Blog and category archives now show a numbered page list (first, last,
current +/-1, collapsing gaps into an ellipsis) instead of only
previous/next — see shared/utils/pagination.ts's paginationRange().

Renders page links via NuxtLink's custom/v-slot API rather than letting
it render its own <a>: RouterLink's built-in active-class/aria-current
detection compares only the route's path, not its query string, so
every ?page=N link was incorrectly marked as the current page.

The language switch now shows both "Italiano / English" at all times,
with the current one as plain non-interactive text and the other as a
link, moved to the header's top-right corner instead of stacked under
the logo.

frontend/app/assets/css/main.css

@@ -94,6 +94,19 @@ h3 {
   color: var(--color-muted);
 }
 
+/* Visually hidden but still announced by screen readers. */
+.sr-only {
+  position: absolute;
+  width: 1px;
+  height: 1px;
+  padding: 0;
+  margin: -1px;
+  overflow: hidden;
+  clip: rect(0, 0, 0, 0);
+  white-space: nowrap;
+  border: 0;
+}
+
 /* Small uppercase label used for categories, dates and section headings. */
 .kicker {
   margin: 0;
@@ -223,16 +236,3 @@ h3 {
   font-size: 0.9em;
 }
 
-.pagination {
-  display: flex;
-  flex-wrap: wrap;
-  gap: 1rem 2rem;
-  align-items: center;
-  justify-content: center;
-  margin-top: var(--gap-section);
-  padding-top: 2rem;
-  border-top: 1px solid var(--color-border);
-  font-size: 0.85rem;
-  letter-spacing: 0.06em;
-  text-transform: uppercase;
-}