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/layouts/default.vue

@@ -11,16 +11,15 @@ const strings = computed(() => STRINGS[locale.value])
     <a class="skip-link" href="#main">{{ strings.skipLink }}</a>
 
     <header class="site-header">
-      <div class="container">
+      <div class="container header-inner">
+        <LanguageSwitch class="lang-switch-header" />
+
         <p class="wordmark">
           <NuxtLink :to="localePath(locale, '/')">
             <img src="/logo.png" :alt="SITE_NAME" width="400" height="400">
           </NuxtLink>
         </p>
         <p class="tagline kicker">{{ strings.tagline }}</p>
-        <p class="lang-switch-header">
-          <LanguageSwitch />
-        </p>
       </div>
 
       <nav class="site-nav" :aria-label="strings.navAriaLabel">
@@ -102,6 +101,16 @@ const strings = computed(() => STRINGS[locale.value])
   }
 }
 
+.header-inner {
+  position: relative;
+}
+
+.lang-switch-header {
+  position: absolute;
+  top: 0;
+  right: var(--space);
+}
+
 .wordmark {
   margin: 0;
 }
@@ -121,10 +130,6 @@ const strings = computed(() => STRINGS[locale.value])
   margin-top: 0.6rem;
 }
 
-.lang-switch-header {
-  margin-top: 0.5rem;
-}
-
 .site-nav {
   margin-top: 1.25rem;
   border-top: 1px solid var(--color-border);