fix: render 404 page even on non-existent pages in /api

[JasonChong96]

Problem

The /api route has no default handler. This causes a 504 error to be shown to users when erroneously accessing a link under the /api route. Even though this is not a common mistake, it should still show the normal 404 page.

Closes #236

Solution

Add a default handler to /api that renders the 404 page.