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/QrButton.svelte

@@ -24,7 +24,7 @@
   onclick={() => (open = true)}
   class={cls}
 >
-  <Icon icon="mdi:qrcode" class="size-[1em]" />
+  <Icon icon="lucide:qr-code" class="size-[1em]" />
 </button>
 
 <QrModal value={open ? value : null} {label} {sublabel} onclose={() => (open = false)} />