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/YoutubeVideoGalleryBlock.svelte

@@ -191,7 +191,7 @@
               <span
                 class="inline-flex h-12 w-12 items-center justify-center rounded-full bg-white/90 text-stein-900 shadow-md group-hover:bg-white"
               >
-                <Icon icon="mdi:play" class="text-2xl" aria-hidden="true" />
+                <Icon icon="lucide:play" class="text-2xl" aria-hidden="true" />
               </span>
             </span>
             {#if label}
@@ -251,7 +251,7 @@
               onclick={modalPrev}
               aria-label={t(T.image_gallery_prev_aria)}
             >
-              <Icon icon="mdi:chevron-left" class="text-2xl" aria-hidden="true" />
+              <Icon icon="lucide:chevron-left" class="text-2xl" aria-hidden="true" />
             </button>
             <button
               type="button"
@@ -259,7 +259,7 @@
               onclick={modalNext}
               aria-label={t(T.image_gallery_next_aria)}
             >
-              <Icon icon="mdi:chevron-right" class="text-2xl" aria-hidden="true" />
+              <Icon icon="lucide:chevron-right" class="text-2xl" aria-hidden="true" />
             </button>
           {/if}
           <button
@@ -268,7 +268,7 @@
             onclick={closeModal}
             aria-label={t(T.image_gallery_close)}
           >
-            <Icon icon="mdi:close" class="text-lg" aria-hidden="true" />
+            <Icon icon="lucide:x" class="text-lg" aria-hidden="true" />
           </button>
         </div>
       </div>
@@ -312,7 +312,7 @@
             aria-label={t(T.image_gallery_prev_aria)}
             onclick={prev}
           >
-            <Icon icon="mdi:chevron-left" class="text-2xl" aria-hidden="true" />
+            <Icon icon="lucide:chevron-left" class="text-2xl" aria-hidden="true" />
           </button>
           <button
             type="button"
@@ -320,7 +320,7 @@
             aria-label={t(T.image_gallery_next_aria)}
             onclick={next}
           >
-            <Icon icon="mdi:chevron-right" class="text-2xl" aria-hidden="true" />
+            <Icon icon="lucide:chevron-right" class="text-2xl" aria-hidden="true" />
           </button>
         {/if}
       </div>