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;
+}

web/src/lib/og-types.ts

@@ -16,18 +16,19 @@ export type OGImageSpec =
  * @returns Full URL to the R2-hosted image
  */
 export function getOGImageUrl(spec: OGImageSpec): string {
-  const R2_BASE = import.meta.env.VITE_OG_R2_BASE_URL;
+	const R2_BASE = import.meta.env.VITE_OG_R2_BASE_URL;
 
-  if (!R2_BASE) {
-    throw new Error("VITE_OG_R2_BASE_URL environment variable is not set");
-  }
+	if (!R2_BASE) {
+		// During prerendering or development, use a fallback placeholder
+		return "/og/placeholder.png";
+	}
 
-  switch (spec.type) {
-    case "index":
-      return `${R2_BASE}/og/index.png`;
-    case "projects":
-      return `${R2_BASE}/og/projects.png`;
-    case "project":
-      return `${R2_BASE}/og/project/${spec.id}.png`;
-  }
+	switch (spec.type) {
+		case "index":
+			return `${R2_BASE}/og/index.png`;
+		case "projects":
+			return `${R2_BASE}/og/projects.png`;
+		case "project":
+			return `${R2_BASE}/og/project/${spec.id}.png`;
+	}
 }

web/src/routes/+error.svelte

@@ -0,0 +1,38 @@
+<script lang="ts">
+  import AppWrapper from "$lib/components/AppWrapper.svelte";
+  import { page } from "$app/stores";
+
+  const status = $derived($page.status);
+  
+  const messages: Record<number, string> = {
+    404: "Page not found",
+    405: "Method not allowed",
+    500: "Something went wrong",
+    502: "Service temporarily unavailable",
+    503: "Service temporarily unavailable",
+  };
+
+  const message = $derived(messages[status] || "An error occurred");
+  const showHomeLink = $derived(![502, 503].includes(status));
+</script>
+
+<svelte:head>
+  <title>{status} - {message}</title>
+</svelte:head>
+
+<AppWrapper>
+  <div class="flex min-h-screen items-center justify-center">
+    <div class="mx-4 max-w-2xl text-center">
+      <h1 class="mb-4 font-hanken text-8xl text-zinc-200">{status}</h1>
+      <p class="mb-8 text-2xl text-zinc-400">{message}</p>
+      {#if showHomeLink}
+        <a
+          href="/"
+          class="inline-block rounded-sm bg-zinc-900 px-4 py-2 text-zinc-100 transition-colors hover:bg-zinc-800"
+        >
+          Return home
+        </a>
+      {/if}
+    </div>
+  </div>
+</AppWrapper>

web/src/routes/errors/[code]/+layout.server.ts

@@ -0,0 +1,2 @@
+// Reset layout chain - no server-side data loading for error pages
+export const prerender = true;

web/src/routes/errors/[code]/+page.server.ts

@@ -0,0 +1,25 @@
+import { ERROR_CODES } from "$lib/error-codes";
+import type { EntryGenerator, PageServerLoad } from "./$types";
+
+/**
+ * Tell SvelteKit to prerender all defined error codes.
+ * This generates static HTML files at build time.
+ */
+export const entries: EntryGenerator = () => {
+	return Object.keys(ERROR_CODES).map((code) => ({ code }));
+};
+
+export const prerender = true;
+
+/**
+ * Load error metadata for the page.
+ * This runs during prerendering to generate static HTML.
+ */
+export const load: PageServerLoad = ({ params }) => {
+	const code = parseInt(params.code, 10) as keyof typeof ERROR_CODES;
+
+	return {
+		code,
+		...ERROR_CODES[code],
+	};
+};

web/src/routes/errors/[code]/+page.svelte

@@ -0,0 +1,38 @@
+<script lang="ts">
+  import { resolve } from "$app/paths";
+  import AppWrapper from "$components/AppWrapper.svelte";
+  import Dots from "$lib/components/Dots.svelte";
+
+  let { data } = $props();
+
+  const title = $derived(
+    `${data.code} - ${data.message.charAt(0).toUpperCase() + data.message.slice(1)}`,
+  );
+</script>
+
+<svelte:head>
+  <title>{title}</title>
+</svelte:head>
+
+<AppWrapper>
+  <div class="min-h-screen flex items-center justify-center">
+    <div class="mx-4 max-w-3xl text-center">
+      <h1 class="text-6xl sm:text-9xl font-hanken font-black text-zinc-200">
+        {data.code}
+      </h1>
+      <p class="text-2xl sm:text-3xl text-zinc-400 mb-8 capitalize">
+        {data.message}
+      </p>
+
+      <!-- Only show "Return home" for non-transient errors -->
+      {#if !data.transient}
+        <a
+          href={resolve("/")}
+          class="inline-block py-2 px-4 bg-zinc-900 text-zinc-50 no-underline rounded-sm transition-colors hover:bg-zinc-800"
+        >
+          Return home
+        </a>
+      {/if}
+    </div>
+  </div>
+</AppWrapper>