fix(win): route shell-outs through a windowless helper (no cmd.exe flash)

_popen/_popen-style shell-outs flash a cmd.exe console window on Windows. Add
Platform::runHiddenCapture() — CreateProcess + CREATE_NO_WINDOW capturing stdout on
Windows, popen on POSIX — and route the remaining shell-outs through it:
- GPU-aware idle detection (getGpuUtilization: "where nvidia-smi" / "nvidia-smi --query-gpu")
- xmrig discovery + version (findXmrigBinary "where xmrig.exe"; "<bin> --version", stderr merged)
- wallet-rebuild helper (app_network) — keeps its exit-code check via the new exitCode out-param

None of these are on the launch path (that was the daemon spawn, fixed in a2f84be); each
would flash a console only when it ran (idle-GPU mining, mining tab, wallet recovery).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/daemon/xmrig_manager.cpp

@@ -23,6 +23,7 @@
 #include <curl/curl.h>
 
 #include "../util/logger.h"
+#include "../util/platform.h"
 #include "../util/pool_registry.h"
 
 #ifdef _WIN32
@@ -145,32 +146,18 @@ std::string XmrigManager::findXmrigBinary() {
         return path;
     }
 
-    // Fallback: system PATH
+    // Fallback: system PATH — windowless so it never flashes a console.
 #ifdef _WIN32
-    FILE* f = _popen("where xmrig.exe 2>nul", "r");
+    std::string out = util::Platform::runHiddenCapture("where xmrig.exe");
 #else
-    FILE* f = popen("which xmrig 2>/dev/null", "r");
-#endif
-    if (f) {
-        char line[512];
-        if (fgets(line, sizeof(line), f)) {
-            std::string s(line);
-            while (!s.empty() && (s.back() == '\n' || s.back() == '\r'))
-                s.pop_back();
-            if (!s.empty() && fs::exists(s)) {
-#ifdef _WIN32
-                _pclose(f);
-#else
-                pclose(f);
-#endif
-                return s;
-            }
-        }
-#ifdef _WIN32
-        _pclose(f);
-#else
-        pclose(f);
+    std::string out = util::Platform::runHiddenCapture("which xmrig");
 #endif
+    {
+        std::string s = out;
+        const auto nl = s.find_first_of("\r\n");   // first line only
+        if (nl != std::string::npos) s.erase(nl);
+        while (!s.empty() && (s.back() == ' ' || s.back() == '\t')) s.pop_back();
+        if (!s.empty() && fs::exists(s)) return s;
     }
 
     return {};
@@ -927,24 +914,10 @@ void XmrigManager::startVersionDetection()
         const bool binShellSafe =
             !bin.empty() && bin.find_first_of("\"'`$;&|<>^%\n\r") == std::string::npos;
         if (binShellSafe) {
-            const std::string cmd = "\"" + bin + "\" --version 2>&1";
-#ifdef _WIN32
-            FILE* fp = _popen(cmd.c_str(), "r");
-#else
-            FILE* fp = popen(cmd.c_str(), "r");
-#endif
-            if (fp) {
-                std::string out;
-                char buf[256];
-                size_t n;
-                while ((n = fread(buf, 1, sizeof(buf), fp)) > 0) out.append(buf, n);
-#ifdef _WIN32
-                _pclose(fp);
-#else
-                pclose(fp);
-#endif
-                ver = parseMinerVersion(out);
-            }
+            // Windowless capture (mergeStderr: xmrig may print --version to stderr) — never flashes.
+            const std::string cmd = "\"" + bin + "\" --version";
+            const std::string out = util::Platform::runHiddenCapture(cmd, /*mergeStderr=*/true);
+            if (!out.empty()) ver = parseMinerVersion(out);
         }
         std::lock_guard<std::mutex> lk(g_installed_ver_mutex);
         g_installed_ver = ver;