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/Accordion.svelte

@@ -34,7 +34,7 @@
 
 <details {id} {open} ontoggle={onToggle} class="pt-4 border-t border-stein-200 group {cls}">
   <summary class="flex cursor-pointer list-none items-center gap-2 text-sm font-medium select-none">
-    <Icon icon="mdi:chevron-right" class="size-4 shrink-0 transition-transform group-open:rotate-90" />
+    <Icon icon="lucide:chevron-right" class="size-4 shrink-0 transition-transform group-open:rotate-90" />
     {label}
   </summary>
   <div class="mt-4">