Missing or expired paste IDs (and unknown routes) now render the main
UI chrome with a 'Paste ID not found' message in a color-coded result
card instead of a bare text/JSON 404. HTTP status stays 404. No inline
scripts or styles; new CSS uses existing --err token and pill radii.
Missing/expired/burned paste IDs and unknown routes now render the main
UI (topbar + centered result card) with a 'Paste ID not found' message,
returning HTTP 404 status for correctness.