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/zh.json

@@ -654,6 +654,7 @@
     "show_qr_code": "显示二维码",
     "showing_transactions": "显示第 %d–%d 笔，共 %d 笔交易（总计：%zu）",
     "simple_background": "简单背景",
+    "slider_off": "关闭",
     "start_mining": "开始挖矿",
     "status": "状态",
     "stop_external": "停止外部守护进程",
@@ -670,6 +671,12 @@
     "time_hours_ago": "%d 小时前",
     "time_minutes_ago": "%d 分钟前",
     "time_seconds_ago": "%d 秒前",
+    "timeout_15min": "15分钟",
+    "timeout_1hour": "1小时",
+    "timeout_1min": "1分钟",
+    "timeout_30min": "30分钟",
+    "timeout_5min": "5分钟",
+    "timeout_off": "关闭",
     "to": "至",
     "to_upper": "至",
     "tools": "工具",