Add SHD themed error handling

docs/DESIGN_SYSTEM.md

@@ -13,6 +13,24 @@ This document defines the visual language for SHD.build. New pages should feel l
    underlines. Communicate hover and focus through SHD accent color, borders,
    brightness, and the required focus-visible outline.
 
+### Routed error console
+
+Expected HTTP errors and unexpected server faults use one shared SHD exception
+console inside the normal application shell. The console always displays the
+numeric status, an operational title, a safe explanation, request path, node,
+and context-appropriate recovery controls. Common statuses receive distinct
+language: authentication for 401, clearance for 403, lost location for 404,
+conflict for 409, saturation for 429, and service/fault language for 5xx.
+
+Orange identifies recoverable request errors and actions; red identifies server
+faults. Never display stack traces, database messages, credentials, request
+queries, or raw unexpected exceptions to users. Unexpected server faults receive
+a short reference ID from the global error handler for support correlation while
+the complete error remains server-side. If the root application shell itself
+cannot render, `src/error.html` provides a dependency-free SHD fallback surface.
+Error controls retain square geometry, focus-visible treatment, pressed motion,
+plain-language labels, and stacked mobile recovery actions.
+
 ## Tokens
 
 The canonical tokens live in `src/app.css` under `:root`.