#173: render friendly 'Paste ID not found' UI instead of bare 404

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.

internal/api/server.go

@@ -129,9 +129,12 @@ func (a *apiServer) routes() http.Handler {
 	r.Get("/{id}", a.handlePasteView)
 	r.Post("/{id}", a.handlePasteView)
 
-	r.NotFound(func(w http.ResponseWriter, r *http.Request) {
-		writeErr(w, 404, "not found")
-	})
+// #173: missing paste URLs render the main UI with a friendly not-found
+// message instead of a bare JSON 404. Known routes (above) handle real pages;
+// anything else is a nonexistent paste ID or typo.
+r.NotFound(func(w http.ResponseWriter, r *http.Request) {
+	a.webHandlers().HandleNotFoundPage(w, r)
+})
 	return r
 }