feat(mining): "Update miner" button + dialog wiring the xmrig updater

Wires util::XmrigUpdater into the GUI:

- ui/windows/xmrig_download_dialog.h: a modal (mirrors BootstrapDownloadDialog) that drives
  the updater — Checking -> Up-to-date/Update-available -> Downloading/Verifying/Extracting ->
  Done/Failed, with a progress bar and a "verified against its published checksum" note. On
  success it persists the installed release tag to settings. Rendered each frame from App::render.
- mining_tab: an "Update miner…" button in the pool section, disabled (with a tooltip) while
  xmrig is running so a live binary is never replaced.
- settings: persist the installed DRG-XMRig tag (xmrig_version) for update detection.

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

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

src/config/settings.cpp

@@ -251,6 +251,7 @@ bool Settings::load(const std::string& path)
         if (j.contains("pool_hugepages"))  pool_hugepages_ = j["pool_hugepages"].get<bool>();
         if (j.contains("pool_mode"))       pool_mode_      = j["pool_mode"].get<bool>();
         if (j.contains("mine_when_idle"))   mine_when_idle_ = j["mine_when_idle"].get<bool>();
+        if (j.contains("xmrig_version"))    xmrig_version_  = j["xmrig_version"].get<std::string>();
         if (j.contains("mine_idle_delay"))  mine_idle_delay_= std::max(30, j["mine_idle_delay"].get<int>());
         if (j.contains("idle_thread_scaling"))  idle_thread_scaling_ = j["idle_thread_scaling"].get<bool>();
         if (j.contains("idle_threads_active")) idle_threads_active_ = j["idle_threads_active"].get<int>();
@@ -383,6 +384,7 @@ bool Settings::save(const std::string& path)
     j["pool_hugepages"] = pool_hugepages_;
     j["pool_mode"]      = pool_mode_;
     j["mine_when_idle"] = mine_when_idle_;
+    j["xmrig_version"]  = xmrig_version_;
     j["mine_idle_delay"]= mine_idle_delay_;
     j["idle_thread_scaling"]  = idle_thread_scaling_;
     j["idle_threads_active"] = idle_threads_active_;