feat(header): icon nav redesign + migrate icons mdi→lucide

- Header desktop nav: social icons grouped in a wald-tinted pill with
  labels, active-state (aria-current + bold), opacity hover (no per-item bg)
- Mobile overlay: social links as icon+label rows with active-state
- Search button: lucide:search, icon-only
- Migrate all icons mdi→lucide app-wide (234 refs); brand/language logos
  (google, whatsapp, mastodon, telegram, language-*) stay on mdi
- generate-icons: emit mdi + lucide subsets; add @iconify-json/lucide
- iconify-offline: register lucide collection
- scripts/icon-map|validate|apply: reusable migration tooling

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/lib/components/blocks/CalendarCompactBlock.svelte

@@ -69,7 +69,7 @@
   data-block-slug={block._slug}
 >
   <header class="flex items-center gap-2 border-b border-stein-100 px-3 py-2">
-    <Icon icon="mdi:calendar-month-outline" class="size-8 shrink-0 text-stein-900" aria-hidden="true" />
+    <Icon icon="lucide:calendar" class="size-8 shrink-0 text-stein-900" aria-hidden="true" />
     <h2 class="text-sm font-semibold leading-none text-stein-800">
       {block.title ?? t(T.calendar_next_events)}
     </h2>
@@ -120,7 +120,7 @@
       class="flex items-center justify-center gap-1 border-t border-stein-100 bg-stein-50 px-3 py-1.5 text-xs font-medium text-wald-700 hover:bg-wald-50"
     >
       {t(T.calendar_all_upcoming)}
-      <Icon icon="mdi:arrow-right" class="size-3.5" aria-hidden="true" />
+      <Icon icon="lucide:arrow-right" class="size-3.5" aria-hidden="true" />
     </a>
   {/if}
 </div>