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/de.json

@@ -654,6 +654,7 @@
     "show_qr_code": "QR-Code anzeigen",
     "showing_transactions": "Zeige %d–%d von %d Transaktionen (gesamt: %zu)",
     "simple_background": "Einfacher Hintergrund",
+    "slider_off": "Aus",
     "start_mining": "Mining starten",
     "status": "Status",
     "stop_external": "Externen Daemon stoppen",
@@ -670,6 +671,12 @@
     "time_hours_ago": "vor %d Stunden",
     "time_minutes_ago": "vor %d Minuten",
     "time_seconds_ago": "vor %d Sekunden",
+    "timeout_15min": "15 Min",
+    "timeout_1hour": "1 Stunde",
+    "timeout_1min": "1 Min",
+    "timeout_30min": "30 Min",
+    "timeout_5min": "5 Min",
+    "timeout_off": "Aus",
     "to": "An",
     "to_upper": "AN",
     "tools": "WERKZEUGE",