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-lock.json

@@ -26,6 +26,7 @@
         "sharp": "^0.34.5"
       },
       "devDependencies": {
+        "@iconify-json/lucide": "^1.2.114",
         "@iconify-json/mdi": "^1.2.3",
         "@sveltejs/adapter-node": "^5.2.12",
         "@sveltejs/kit": "^2.65.0",
@@ -610,6 +611,16 @@
         "url": "https://github.com/sponsors/ayuhito"
       }
     },
+    "node_modules/@iconify-json/lucide": {
+      "version": "1.2.114",
+      "resolved": "https://registry.npmjs.org/@iconify-json/lucide/-/lucide-1.2.114.tgz",
+      "integrity": "sha512-NbvH3B1BYo6wBtS7joLi7f2UVQOqK2dtZodMFf3kkBs+Tnh9TkRuy8oVHr1RM8UK6bUtvAXxfNlGAah0CuvPCw==",
+      "dev": true,
+      "license": "ISC",
+      "dependencies": {
+        "@iconify/types": "*"
+      }
+    },
     "node_modules/@iconify-json/mdi": {
       "version": "1.2.3",
       "resolved": "https://registry.npmjs.org/@iconify-json/mdi/-/mdi-1.2.3.tgz",