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/ko.json

@@ -654,6 +654,7 @@
     "show_qr_code": "QR 코드 표시",
     "showing_transactions": "%d–%d / %d건의 거래 표시 중 (총: %zu)",
     "simple_background": "단순 배경",
+    "slider_off": "끔",
     "start_mining": "채굴 시작",
     "status": "상태",
     "stop_external": "외부 데몬 중지",
@@ -670,6 +671,12 @@
     "time_hours_ago": "%d시간 전",
     "time_minutes_ago": "%d분 전",
     "time_seconds_ago": "%d초 전",
+    "timeout_15min": "15분",
+    "timeout_1hour": "1시간",
+    "timeout_1min": "1분",
+    "timeout_30min": "30분",
+    "timeout_5min": "5분",
+    "timeout_off": "끔",
     "to": "받는 곳",
     "to_upper": "받는 곳",
     "tools": "도구",