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/Search.svelte

@@ -90,7 +90,7 @@
       <label class="relative block max-w-xl mx-auto">
         <span class="sr-only">Suche</span>
         <span class="pointer-events-none absolute top-1/2 left-3 -translate-y-1/2 text-stein-500">
-          <Icon icon="mdi:magnify" class="h-5 w-5" />
+          <Icon icon="lucide:search" class="h-5 w-5" />
         </span>
         <input
           bind:this={inputEl}
@@ -106,7 +106,7 @@
             aria-label="Eingabe leeren"
             class="absolute top-1/2 right-2 -translate-y-1/2 rounded-xs p-1 text-stein-500 hover:bg-stein-100 hover:text-stein-900"
           >
-            <Icon icon="mdi:close" class="h-4 w-4" />
+            <Icon icon="lucide:x" class="h-4 w-4" />
           </button>
         {/if}
       </label>
@@ -138,7 +138,7 @@
                     />
                   {:else}
                     <span class="w-20 h-12 shrink-0 bg-stein-100 rounded-xs border border-stein-200 flex items-center justify-center text-stein-400">
-                      <Icon icon={hit.type === "post" ? "mdi:file-document-outline" : "mdi:file-outline"} class="h-5 w-5" />
+                      <Icon icon={hit.type === "post" ? "lucide:file-text" : "lucide:file"} class="h-5 w-5" />
                     </span>
                   {/if}
                   <div class="min-w-0 flex-1">