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/OrganisationsBlock.svelte

@@ -126,7 +126,7 @@
             onclick={(e) => e.stopPropagation()}
             class="inline-flex items-center gap-1 rounded-sm border border-stein-200 bg-stein-50 px-2 py-0.5 text-[0.6rem] text-stein-600 no-underline transition-colors hover:border-red-300 hover:bg-red-50 hover:text-red-700"
           >
-            <Icon icon="mdi:youtube" class="size-3 shrink-0" />
+            <Icon icon="lucide:youtube" class="size-3 shrink-0" />
             <span>YouTube</span>
           </a>
         {/if}
@@ -198,7 +198,7 @@
           aria-label="Vorherige Organisationen"
           class="hidden md:flex absolute -left-3 top-1/2 -translate-y-1/2 -translate-x-1/2 size-7 items-center justify-center rounded-full bg-white border border-stein-200 shadow-md hover:bg-stein-50 transition"
         >
-          <Icon icon="mdi:chevron-left" class="size-4" aria-hidden="true" />
+          <Icon icon="lucide:chevron-left" class="size-4" aria-hidden="true" />
         </button>
         <button
           type="button"
@@ -206,7 +206,7 @@
           aria-label="Weitere Organisationen"
           class="hidden md:flex absolute -right-3 top-1/2 -translate-y-1/2 translate-x-1/2 size-7 items-center justify-center rounded-full bg-white border border-stein-200 shadow-md hover:bg-stein-50 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>