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)

src/util/i18n.cpp

@@ -33,13 +33,13 @@ I18n::I18n()
     // Cyrillic glyphs are included in the Ubuntu font glyph ranges.
     registerLanguage("en", "English");
     registerLanguage("es", "Español");
-    registerLanguage("zh", "Chinese");
-    registerLanguage("ru", "Russian");
+    registerLanguage("zh", "中文");
+    registerLanguage("ru", "Русский");
     registerLanguage("de", "Deutsch");
     registerLanguage("fr", "Français");
     registerLanguage("pt", "Português");
-    registerLanguage("ja", "Japanese");
-    registerLanguage("ko", "Korean");
+    registerLanguage("ja", "日本語");
+    registerLanguage("ko", "한국어");
     
     // Load default English strings (built-in fallback)
     loadLanguage("en");