Proxy "Not found" route should supply more information

### Problem description

Proxy currently shows a short "Not found" message when invalid/non-existent route is displayed.

Since Proxy is an edge component often exposed via various reverse-proxy solutions it may be non-obvious where the error comes from.

### Proposed solution

Implement proper 404 page so that it's obvious that the error comes from Defguard Proxy.

### Alternatives considered

Redirect to the main page

### Impact

Nice to have