feat(mining): move "Update miner" into the benchmark row, showing latest + current version

Relocate the miner-update control from a standalone full-width button into the mining-control
header row, immediately left of the benchmark button:
- The button now shows the latest available version ("Update <tag>"), with the current installed
  version as text to its left ("Current: <tag>" / "none").
- A one-shot background version check (util::XmrigUpdater::startCheck) runs the first time the pool
  section is shown, so the latest tag can be displayed; until it arrives the button reads
  "Update miner…". Clicking opens the existing dialog; disabled (greyed, with tooltip) while the
  miner is running.
- New i18n keys: xmrig_update_short, xmrig_current, xmrig_none.

Both variants build; suite passes; GUI smoke-launched without crash.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/util/i18n.cpp

@@ -1038,6 +1038,9 @@ void I18n::loadBuiltinEnglish()
 
         // --- Miner (xmrig) updater ---
         strings_["xmrig_update_button"]      = "Update miner…";
+        strings_["xmrig_update_short"]       = "Update";
+        strings_["xmrig_current"]            = "Current:";
+        strings_["xmrig_none"]               = "none";
         strings_["xmrig_update_title"]       = "Update Miner";
         strings_["xmrig_stop_mining_first"]  = "Stop mining before updating the miner.";
         strings_["xmrig_checking"]           = "Checking for the latest miner…";