fix(lite): probe servers without joining the thread on the UI thread

Adding a lite server from the network tab restarted the background
server probe immediately. Starting a probe while one is still running
joins the previous worker thread, and that join happened on the UI
thread, so the frame could stall until the outstanding probe finished.

- Adding a server now sets a dirty flag instead of restarting the probe
  inline; the next frame starts a fresh probe once the current pass has
  finished, so the UI thread never blocks on a join.
- The probe's libcurl transfers honor the cancel flag through a progress
  callback and abort cleanly, so a probe in flight stops promptly.

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

src/util/lite_server_probe.cpp

@@ -8,11 +8,17 @@
 
 #include <curl/curl.h>
 
+#include <atomic>
 #include <utility>
 
 namespace dragonx {
 namespace util {
 
+static int liteProbeXferCb(void* p, curl_off_t, curl_off_t, curl_off_t, curl_off_t)
+{
+    return static_cast<const std::atomic<bool>*>(p)->load(std::memory_order_relaxed) ? 1 : 0;
+}
+
 std::string liteServerHost(const std::string& url)
 {
     // Strip scheme ("scheme://") then take everything up to the first '/', '?' or '#'.
@@ -69,7 +75,12 @@ void LiteServerProbe::run(std::vector<std::string> urls)
             curl_easy_setopt(c, CURLOPT_SSL_VERIFYPEER, 1L);
             curl_easy_setopt(c, CURLOPT_SSL_VERIFYHOST, 2L);
 
+            curl_easy_setopt(c, CURLOPT_NOPROGRESS, 0L);
+            curl_easy_setopt(c, CURLOPT_XFERINFOFUNCTION, liteProbeXferCb);
+            curl_easy_setopt(c, CURLOPT_XFERINFODATA, const_cast<std::atomic<bool>*>(&cancel_));
+
             const CURLcode res = curl_easy_perform(c);
+            if (res == CURLE_ABORTED_BY_CALLBACK) { curl_easy_cleanup(c); break; }
             if (res == CURLE_OK) {
                 r.online = true;
                 double t = 0.0;