feat: Align UI to brand guidelines — Oxide Orange #F26622 replaces red accent

Defines oxide color scale (50-950) via Tailwind v4 @theme directive.
Swaps all accent red-* classes to oxide-* across layouts, auth views,
and footer. Semantic reds (error banners, offline status, stop button)
intentionally preserved.

Brand spec: Corrosion_Management_Brand_Guidelines.pdf

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/components/layout/PublicLayout.vue

@@ -9,7 +9,7 @@ import { RouterView } from 'vue-router'
     <footer class="py-6 text-center text-neutral-600 text-sm border-t border-neutral-800">
       <div class="flex items-center justify-center gap-2">
         <img src="/logo.png" alt="Corrosion" class="h-4 w-4 opacity-60" />
-        <span>Powered by <span class="text-red-500 font-semibold">Corrosion</span></span>
+        <span>Powered by <span class="text-oxide-500 font-semibold">Corrosion</span></span>
       </div>
     </footer>
   </div>