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/ui/windows/mining_tab.cpp

@@ -6,6 +6,7 @@
 #include "mining_benchmark.h"
 #include "mining_tab_helpers.h"
 #include "mining_pool_panel.h"
+#include "xmrig_download_dialog.h"
 #include "../../app.h"
 #include "../../util/i18n.h"
 #include "../../config/version.h"
@@ -1574,6 +1575,21 @@ static void RenderMiningTabContent(App* app)
         ImGui::Dummy(ImVec2(0, gap));
     }
 
+    // ================================================================
+    // POOL MINER — download / update the bundled DRG-XMRig miner from the project release server.
+    // (Pool mode uses xmrig; disabled while it's running so we never replace a live binary.)
+    // ================================================================
+    if (s_pool_mode) {
+        const bool minerBusy = state.pool_mining.xmrig_running;
+        ImGui::BeginDisabled(minerBusy);
+        if (TactileButton("Update miner…", ImVec2(availWidth, 0)))
+            XmrigDownloadDialog::show(app);
+        ImGui::EndDisabled();
+        if (minerBusy && ImGui::IsItemHovered(ImGuiHoveredFlags_AllowWhenDisabled))
+            ImGui::SetTooltip("Stop mining before updating the miner.");
+        ImGui::Dummy(ImVec2(0, gap));
+    }
+
     // ================================================================
     // HASHRATE + STATS — Combined glass card: stat values on top, chart below
     // (Or full-card log view when toggled in pool mode)