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/QrModal.svelte

@@ -71,7 +71,7 @@
         class="absolute right-3 top-3 text-stein-400 hover:text-stein-700"
         aria-label="Schließen"
       >
-        <Icon icon="mdi:close" class="size-5" />
+        <Icon icon="lucide:x" class="size-5" />
       </button>
       {#if label}
         <p class="mb-1 text-sm font-medium text-stein-900">{label}</p>
@@ -93,7 +93,7 @@
         disabled={!qrPngUrl}
         class="btn-outline btn-sm mt-3 w-full justify-center"
       >
-        <Icon icon="mdi:download" class="size-3.5" />
+        <Icon icon="lucide:download" class="size-3.5" />
         Als PNG herunterladen
       </button>
     </div>