feat: add prerendered error pages with Rust integration

- Prerender 20+ HTTP error codes (4xx/5xx) at build time
- Embed error pages in binary and serve via Axum
- Intercept error responses for HTML requests
- Add transient flag for retry-worthy errors (502/503/504)

web/src/lib/error-codes.ts

@@ -0,0 +1,41 @@
+/**
+ * Single source of truth for all HTTP error codes.
+ * Used by:
+ * - SvelteKit EntryGenerator (prerendering)
+ * - Error page components (rendering)
+ * - Rust assets.rs (validation only)
+ */
+export const ERROR_CODES = {
+	// 4xx Client Errors
+	400: { message: "Bad request", transient: false },
+	401: { message: "Unauthorized", transient: false },
+	403: { message: "Forbidden", transient: false },
+	404: { message: "Page not found", transient: false },
+	405: { message: "Method not allowed", transient: false },
+	406: { message: "Not acceptable", transient: false },
+	408: { message: "Request timeout", transient: true },
+	409: { message: "Conflict", transient: false },
+	410: { message: "Gone", transient: false },
+	413: { message: "Payload too large", transient: false },
+	414: { message: "URI too long", transient: false },
+	415: { message: "Unsupported media type", transient: false },
+	418: { message: "I'm a teapot", transient: false }, // RFC 2324 Easter egg
+	422: { message: "Unprocessable entity", transient: false },
+	429: { message: "Too many requests", transient: true },
+	451: { message: "Unavailable for legal reasons", transient: false },
+
+	// 5xx Server Errors
+	500: { message: "Internal server error", transient: false },
+	501: { message: "Not implemented", transient: false },
+	502: { message: "Bad gateway", transient: true },
+	503: { message: "Service unavailable", transient: true },
+	504: { message: "Gateway timeout", transient: true },
+	505: { message: "HTTP version not supported", transient: false },
+} as const;
+
+export type ErrorCode = keyof typeof ERROR_CODES;
+
+// Helper to check if error code is defined
+export function isDefinedErrorCode(code: number): code is ErrorCode {
+	return code in ERROR_CODES;
+}