fix: accurate sync speed display, add missing i18n keys, native language names

- Fix blk/s calculation that was inflated ~10x due to resetting the
  time baseline every frame instead of only when blocks advanced
- Add decay when no new blocks arrive for 10s so rate doesn't stay stale
- Add 7 missing translation keys (timeout_off/1min/5min/15min/30min/1hour,
  slider_off) to all 8 language files so settings dropdowns translate
- Show language names in native script (中文, Русский, 日本語, 한국어)

res/lang/pt.json

@@ -654,6 +654,7 @@
     "show_qr_code": "Mostrar Código QR",
     "showing_transactions": "Mostrando %d–%d de %d transações (total: %zu)",
     "simple_background": "Fundo simples",
+    "slider_off": "Desligado",
     "start_mining": "Iniciar Mineração",
     "status": "Status",
     "stop_external": "Parar daemon externo",
@@ -670,6 +671,12 @@
     "time_hours_ago": "há %d horas",
     "time_minutes_ago": "há %d minutos",
     "time_seconds_ago": "há %d segundos",
+    "timeout_15min": "15 min",
+    "timeout_1hour": "1 hora",
+    "timeout_1min": "1 min",
+    "timeout_30min": "30 min",
+    "timeout_5min": "5 min",
+    "timeout_off": "Desligado",
     "to": "Para",
     "to_upper": "PARA",
     "tools": "FERRAMENTAS",