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/DeadlineBannerBlock.svelte

@@ -130,7 +130,7 @@
                         class="font-semibold whitespace-nowrap text-gray-900 hover:text-gray-700"
                     >
                         {ctaLabel}&nbsp;<Icon
-                            icon="mdi:arrow-right"
+                            icon="lucide:arrow-right"
                             class="inline-block size-4 align-[-0.15em]"
                             aria-hidden="true"
                         />
@@ -145,7 +145,7 @@
                 >
                     <span class="sr-only">Schließen</span>
                     <Icon
-                        icon="mdi:close"
+                        icon="lucide:x"
                         class="size-5 text-gray-900"
                         aria-hidden="true"
                     />