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/ja.json

@@ -654,6 +654,7 @@
     "show_qr_code": "QRコードを表示",
     "showing_transactions": "%d–%d / %d 件の取引を表示中（合計：%zu）",
     "simple_background": "シンプル背景",
+    "slider_off": "オフ",
     "start_mining": "マイニング開始",
     "status": "ステータス",
     "stop_external": "外部デーモンを停止",
@@ -670,6 +671,12 @@
     "time_hours_ago": "%d時間前",
     "time_minutes_ago": "%d分前",
     "time_seconds_ago": "%d秒前",
+    "timeout_15min": "15分",
+    "timeout_1hour": "1時間",
+    "timeout_1min": "1分",
+    "timeout_30min": "30分",
+    "timeout_5min": "5分",
+    "timeout_off": "オフ",
     "to": "宛先",
     "to_upper": "宛先",
     "tools": "ツール",