web: friendly Paste ID not found page for missing pastes (#173)

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.

internal/api/main_test.go

@@ -299,3 +299,27 @@ func TestNotFound(t *testing.T) {
 		t.Fatalf("expected 404, got %d", rec.Code)
 	}
 }
+
+// #173: a missing paste ID on the UI route (/p/{id}, i.e. /{id} HTML view)
+// should render the main UI page with a friendly "Paste ID not found"
+// message, not a bare text 404. Status stays 404.
+func TestPasteViewNotFoundFriendly(t *testing.T) {
+	s := testServer(t)
+	h := s.routes()
+	req := httptest.NewRequest("GET", "/zzzzzz", nil)
+	rec := httptest.NewRecorder()
+	h.ServeHTTP(rec, req)
+	if rec.Code != http.StatusNotFound {
+		t.Fatalf("expected 404 status, got %d", rec.Code)
+	}
+	body := rec.Body.String()
+	if !strings.Contains(body, "Paste ID not found") {
+		t.Fatalf("expected friendly message in body, got: %.200s", body)
+	}
+	if !strings.Contains(body, "<nav>") {
+		t.Fatalf("expected main UI chrome in body")
+	}
+	if strings.Contains(body, "404 page not found") {
+		t.Fatalf("body still contains bare Go 404 text")
+	}
+}