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/DashboardLayout.vue

@@ -62,7 +62,7 @@ function handleLogout() {
         <div class="flex items-center gap-3">
           <img src="/logo.png" alt="Corrosion" class="h-8 w-8" />
           <div>
-            <h1 class="text-sm font-bold text-red-500 tracking-wider">CORROSION</h1>
+            <h1 class="text-sm font-bold text-oxide-500 tracking-wider">CORROSION</h1>
             <p class="text-xs text-neutral-500">{{ auth.license?.server_name || 'Server Management' }}</p>
           </div>
         </div>
@@ -93,7 +93,7 @@ function handleLogout() {
           :to="item.path"
           class="flex items-center gap-3 px-4 py-2 mx-2 rounded-lg text-sm transition-colors"
           :class="isActive(item.path)
-            ? 'bg-red-500/10 text-red-400'
+            ? 'bg-oxide-500/10 text-oxide-400'
             : 'text-neutral-400 hover:bg-neutral-800 hover:text-neutral-200'"
         >
           <component :is="item.icon" class="w-4 h-4" />
@@ -110,7 +110,7 @@ function handleLogout() {
           </div>
           <button
             @click="handleLogout"
-            class="text-neutral-500 hover:text-red-400 transition-colors"
+            class="text-neutral-500 hover:text-oxide-400 transition-colors"
           >
             <LogOut class="w-4 h-4" />
           </button>