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/SocialImageModal.svelte

@@ -100,7 +100,7 @@
             title="Direktlink zu dieser Vorschau kopieren"
             aria-label="Direktlink kopieren"
           >
-            <Icon icon={linkCopied ? "mdi:check" : "mdi:link-variant"} class="size-3.5" />
+            <Icon icon={linkCopied ? "lucide:check" : "lucide:link"} class="size-3.5" />
             {linkCopied ? "Kopiert" : "Direktlink"}
           </button>
           <button
@@ -109,7 +109,7 @@
             class="text-stein-400 hover:text-stein-700"
             aria-label="Schließen"
           >
-            <Icon icon="mdi:close" class="size-5" />
+            <Icon icon="lucide:x" class="size-5" />
           </button>
         </div>
       </div>
@@ -170,7 +170,7 @@
             />
           </div>
           <a href={ogUrl} download={`${downloadPrefix}-${slug}-og-${theme}.png`} class="btn-outline btn-sm justify-center no-underline!">
-            <Icon icon="mdi:download" class="size-3.5" />
+            <Icon icon="lucide:download" class="size-3.5" />
             Quer herunterladen
           </a>
         </div>
@@ -189,7 +189,7 @@
             />
           </div>
           <a href={sqUrl} download={`${downloadPrefix}-${slug}-square-${theme}.png`} class="btn-outline btn-sm justify-center no-underline!">
-            <Icon icon="mdi:download" class="size-3.5" />
+            <Icon icon="lucide:download" class="size-3.5" />
             Quadrat herunterladen
           </a>
         </div>
@@ -208,7 +208,7 @@
             />
           </div>
           <a href={storyUrl} download={`${downloadPrefix}-${slug}-story-${theme}.png`} class="btn-outline btn-sm justify-center no-underline!">
-            <Icon icon="mdi:download" class="size-3.5" />
+            <Icon icon="lucide:download" class="size-3.5" />
             Story herunterladen
           </a>
         </div>