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/data/wallet_state.h

@@ -181,7 +181,8 @@ struct WalletState {
     // Connection
     bool connected = false;
     bool warming_up = false;         // daemon reachable but in RPC warmup (error -28)
-    std::string warmup_status;       // e.g. "Activating best chain..."
+    std::string warmup_status;       // user-friendly title, e.g. "Processing blocks..."
+    std::string warmup_description;  // subtitle explaining the stage
     int daemon_version = 0;
     std::string daemon_subversion;
     int protocol_version = 0;
@@ -254,6 +255,7 @@ struct WalletState {
         connected = false;
         warming_up = false;
         warmup_status.clear();
+        warmup_description.clear();
         daemon_version = 0;
         daemon_subversion.clear();
         protocol_version = 0;