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/InfoAccordion.svelte

@@ -17,7 +17,7 @@
     body = "",
     variant = "stein",
     open = false,
-    iconName = "mdi:information-outline",
+    iconName = "lucide:info",
   }: {
     title: string;
     body?: string;
@@ -48,7 +48,7 @@
   <summary class="flex cursor-pointer items-center gap-1.5 px-3 py-1.5 font-medium leading-snug list-none {p.summary}">
     <Icon icon={iconName} class="size-3.5 shrink-0 {p.icon}" />
     <span class="flex-1">{title}</span>
-    <Icon icon="mdi:chevron-down" class="info-accordion-chevron size-3.5 shrink-0 transition-transform {p.icon}" />
+    <Icon icon="lucide:chevron-down" class="info-accordion-chevron size-3.5 shrink-0 transition-transform {p.icon}" />
   </summary>
   <div class="markdown prose prose-sm prose-zinc max-w-none border-t px-3 py-2 leading-relaxed {p.body}">{@html body}</div>
 </details>