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/style.css

@@ -1,12 +1,25 @@
 @import "tailwindcss";
 
-/* Corrosion Platform — Custom Styles */
+/* Corrosion Brand — Oxide Orange #F26622 */
+@theme {
+  --color-oxide-50: #FEF3EB;
+  --color-oxide-100: #FDE3D0;
+  --color-oxide-200: #FBC4A1;
+  --color-oxide-300: #F9A472;
+  --color-oxide-400: #F58543;
+  --color-oxide-500: #F26622;
+  --color-oxide-600: #D9541A;
+  --color-oxide-700: #B84315;
+  --color-oxide-800: #933510;
+  --color-oxide-900: #6E280C;
+  --color-oxide-950: #3D1506;
+}
 
 /* Dark mode is default — Rust servers run at night */
 :root {
-  --corrosion-red: #ef4444;
-  --corrosion-orange: #f97316;
-  --corrosion-dark: #0f0f0f;
+  --corrosion-accent: #F26622;
+  --corrosion-dark: #000000;
+  --corrosion-charcoal: #1E2024;
   --corrosion-surface: #1a1a1a;
   --corrosion-border: #2a2a2a;
 }