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/PostCard.svelte

@@ -102,7 +102,7 @@
           aria-label={t(T.comments_count_aria, { count: commentCount })}
           title={t(T.comments_count_aria, { count: commentCount })}
         >
-          <Icon icon="mdi:comment-outline" class="size-3" />
+          <Icon icon="lucide:message-circle" class="size-3" />
           {commentCount}
         </span>
       </div>