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/ui/Breadcrumbs.svelte

@@ -17,7 +17,7 @@
             aria-label={i === 0 ? item.label : undefined}
           >
             {#if i === 0}
-              <Icon icon="mdi:home" class="size-4 shrink-0" aria-hidden="true" />
+              <Icon icon="lucide:house" class="size-4 shrink-0" aria-hidden="true" />
             {:else}
               <span>{item.label}</span>
             {/if}
@@ -32,7 +32,7 @@
         {/if}
         {#if i < items.length - 1}
           <span aria-hidden="true" class="shrink-0 text-stein-300">
-            <Icon icon="mdi:chevron-right" class="size-3.5" />
+            <Icon icon="lucide:chevron-right" class="size-3.5" />
           </span>
         {/if}
       </li>