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/app.cpp

@@ -46,6 +46,7 @@
 #include "ui/windows/address_label_dialog.h"
 #include "ui/windows/address_transfer_dialog.h"
 #include "ui/windows/bootstrap_download_dialog.h"
+#include "ui/windows/xmrig_download_dialog.h"
 #include "ui/windows/console_tab.h"
 #include "ui/pages/settings_page.h"
 #include "ui/theme.h"
@@ -1529,6 +1530,7 @@ void App::render()
 
     // Bootstrap download from settings
     ui::BootstrapDownloadDialog::render();
+    ui::XmrigDownloadDialog::render();
 
     // Windows Defender antivirus help dialog
     renderAntivirusHelpDialog();