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/PostActions.svelte

@@ -174,7 +174,7 @@
       title={t(T.comments_jump_to)}
       class="no-underline! inline-flex items-center gap-1 rounded-full border border-stein-200 bg-white px-2.5 py-1 text-xs font-medium text-stein-700! hover:bg-stein-50 hover:border-stein-300 transition-colors"
     >
-      <Icon icon="mdi:comment-outline" class="size-3.5" />
+      <Icon icon="lucide:message-circle" class="size-3.5" />
       <span class="tabular-nums">{commentCount ?? "…"}</span>
     </a>
   {/if}
@@ -189,7 +189,7 @@
         title={t(T.post_action_share)}
       >
         <Icon
-          icon={shared ? "mdi:check" : "mdi:share-variant-outline"}
+          icon={shared ? "lucide:check" : "lucide:share-2"}
           class="size-3.5"
         />
       </button>
@@ -201,7 +201,7 @@
       aria-label={copied ? t(T.post_action_copied) : t(T.post_action_copy)}
       title={copied ? t(T.post_action_copied) : t(T.post_action_copy)}
     >
-      <Icon icon={copied ? "mdi:check" : "mdi:link-variant"} class="size-3.5" />
+      <Icon icon={copied ? "lucide:check" : "lucide:link"} class="size-3.5" />
     </button>
     <button
       type="button"
@@ -210,7 +210,7 @@
       aria-label={t(T.post_action_print)}
       title={t(T.post_action_print)}
     >
-      <Icon icon="mdi:printer-outline" class="size-3.5" />
+      <Icon icon="lucide:printer" class="size-3.5" />
     </button>
     {#if socialSlug}
       <button
@@ -220,7 +220,7 @@
         aria-label="Social-Bild Vorschau"
         title="Social-Bild Vorschau (für Instagram, Facebook, WhatsApp)"
       >
-        <Icon icon="mdi:image-outline" class="size-3.5" />
+        <Icon icon="lucide:image" class="size-3.5" />
       </button>
     {/if}
     <QrButton
@@ -247,7 +247,7 @@
       class="inline-flex items-center gap-1 rounded-full border border-stein-200 bg-white px-2.5 py-1 text-xs text-stein-500"
       title={t(T.post_action_reading_time, { minutes: readingTimeMinutes })}
     >
-      <Icon icon="mdi:clock-outline" class="size-3.5" />
+      <Icon icon="lucide:clock" class="size-3.5" />
       <span class="tabular-nums"
         >{t(T.post_action_reading_time, { minutes: readingTimeMinutes })}</span
       >