Fix admin 404 incorrectly falling back to standalone design - #92

Root cause

The full admin error layout was gated behind Auth::guard(...)->check(). Unknown admin URLs have no matching route middleware, so authentication state can be unavailable even for a browser that is signed in, causing the standalone error page shown in the screenshot.

Fix

• Always attempt real Lazy Admin layout for 404, independent of the authentication check.
• Preserve authorization-dependent behavior for 403 and lightweight errors for 401/419/429/500/503.
• Catch failure while rendering the full admin shell and fall back to the standalone page, preserving HTTP 404 instead of escalating the failure.
• Update tests; fix malformed test class namespace escaping.

Single commit. CI / visual verification needed before merging.