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/ImageModal.svelte

@@ -39,6 +39,6 @@
         class="absolute top-4 right-4 p-2 rounded-full bg-black/50 text-white hover:bg-black/70 transition-colors"
         aria-label="Schließen"
     >
-        <Icon icon="mdi:close" class="size-5" aria-hidden="true" />
+        <Icon icon="lucide:x" class="size-5" aria-hidden="true" />
     </button>
 </div>