feat: add NotFound route and tests

[Benedict315]

Summary

Testing

• pnpm lint
• pnpm build

Checklist

• Linked issue or backlog item
• Scope is limited to the stated change
• Updated docs if behavior or setup changed
• Added screenshots for UI changes when relevant
  closes Add integration test for 404 page rendering on unknown routes #427

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[github-actions]

👋 Hey @Benedict315, thanks for your contribution!

This PR is targeting main directly. We use main for stable releases only — all contributions should be opened against the dev branch instead.

Please close this PR and reopen it with dev as the base branch. If you're unsure how to do that, you can change the base branch using the Edit button at the top of this PR page.

Closing this PR automatically. See you in dev! 🚀

[drips-wave]

@Benedict315 Great news! 🎉 Based on an automated assessment of this PR, the linked Wave issue(s) no longer count against your application limits.

You can now already apply to more issues while waiting for a review of this PR. Keep up the great work! 🚀

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