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/ImageGalleryBlock.svelte

@@ -239,7 +239,7 @@
               title={t(T.image_gallery_download)}
               aria-label={t(T.image_gallery_download)}
             >
-              <Icon icon="mdi:download-outline" class="text-xl" aria-hidden="true" />
+              <Icon icon="lucide:download" class="text-xl" aria-hidden="true" />
             </a>
           </div>
           {#if withUrl.length > 1}
@@ -249,7 +249,7 @@
               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"
@@ -257,7 +257,7 @@
               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
@@ -266,7 +266,7 @@
             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>
@@ -337,7 +337,7 @@
         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"
@@ -345,7 +345,7 @@
         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>
 
       <div class="flex justify-center gap-1.5 mt-2" role="tablist" aria-label={t(T.image_gallery_position_aria)}>