BLOG-62 Custom not found page (#63)

### Description

As the title.

### Package Changes

_No response_

### Screenshots

![image.png](/attachments/536ca4c6-1043-46bb-b1f0-1614f8501f9f)

### Reference

Resolves #62

### Checklist

- [x] A milestone is set
- [x] The related issuse has been linked to this branch

Reviewed-on: #63
Co-authored-by: SquidSpirit <squid@squidspirit.com>
Co-committed-by: SquidSpirit <squid@squidspirit.com>

frontend/src/routes/+error.svelte

@@ -0,0 +1,14 @@
+<div
+	class="mx-auto flex min-h-content-height max-w-screen-xl flex-col items-center justify-center px-4 md:px-6"
+>
+	<div class="flex flex-row items-end gap-x-4 md:gap-x-6">
+		<h1 class="text-5xl font-extrabold text-gray-800 underline md:text-7xl">404</h1>
+		<h2 class="flex flex-row items-center gap-x-2 text-2xl font-bold md:gap-x-2.5 md:text-3xl">
+			<div class="h-7 w-1.5 bg-gray-800 md:h-9 md:w-2"></div>
+			<div class="rounded-md bg-blue-600 px-1 py-px md:px-1.5 md:py-0.5">
+				<span class="text-white">Not</span>
+			</div>
+			<span class="text-gray-800">Found.</span>
+		</h2>
+	</div>
+</div>