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/Carousel.svelte

@@ -73,7 +73,7 @@
         aria-label="Vorheriger Eintrag"
         class="shrink-0 inline-flex items-center justify-center size-7 rounded-full bg-white/90 border border-stein-200 shadow-sm hover:bg-white hover:border-wald-700 transition"
       >
-        <Icon icon="mdi:chevron-left" class="size-4" aria-hidden="true" />
+        <Icon icon="lucide:chevron-left" class="size-4" aria-hidden="true" />
       </button>
     {/if}
 
@@ -97,7 +97,7 @@
         aria-label="Nächster Eintrag"
         class="shrink-0 inline-flex items-center justify-center size-7 rounded-full bg-white/90 border border-stein-200 shadow-sm hover:bg-white hover:border-wald-700 transition"
       >
-        <Icon icon="mdi:chevron-right" class="size-4" aria-hidden="true" />
+        <Icon icon="lucide:chevron-right" class="size-4" aria-hidden="true" />
       </button>
     {/if}
   </div>