feat: add marketing landing page for crumbs.bretzel.app

Static landing page with parchment/retro theme matching the app aesthetic.
Includes GitHub Pages deploy workflow, responsive layout, feature grid,
deploy quick-start, and 404 page. Screenshot placeholders ready for
real captures.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

website/404.html

@@ -0,0 +1,40 @@
+<!DOCTYPE html>
+<html lang="en">
+<head>
+	<meta charset="UTF-8">
+	<meta name="viewport" content="width=device-width, initial-scale=1.0">
+	<title>404 - Page not found | Crumbs</title>
+	<link rel="icon" type="image/png" sizes="96x96" href="assets/favicon-96x96.png">
+	<link rel="icon" type="image/x-icon" href="assets/favicon.ico">
+	<link rel="stylesheet" href="styles.css">
+	<style>
+		.error-page {
+			display: flex;
+			flex-direction: column;
+			align-items: center;
+			justify-content: center;
+			min-height: 80vh;
+			text-align: center;
+			padding: 2rem;
+		}
+		.error-code {
+			font-size: clamp(3rem, 10vw, 6rem);
+			color: var(--primary);
+			margin-bottom: 1rem;
+		}
+		.error-message {
+			font-size: 0.85rem;
+			color: var(--text-muted);
+			margin-bottom: 2rem;
+			max-width: 24rem;
+		}
+	</style>
+</head>
+<body>
+	<div class="error-page">
+		<h1 class="error-code font-pixel">404</h1>
+		<p class="error-message">This note seems to have been deleted. Or maybe it never existed.</p>
+		<a href="/" class="btn btn-primary">Back to home</a>
+	</div>
+</body>
+</html>