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>

package.json

@@ -20,6 +20,7 @@
     "prepare": "svelte-kit sync || true"
   },
   "devDependencies": {
+    "@iconify-json/lucide": "^1.2.114",
     "@iconify-json/mdi": "^1.2.3",
     "@sveltejs/adapter-node": "^5.2.12",
     "@sveltejs/kit": "^2.65.0",