perf(daemon): skip the UI-thread datadir lock wait

Starting the daemon polls for an already-running node holding the
datadir, sleeping up to three 100 ms intervals to let a shutting-down
node clear out. When a start runs on the UI thread, that is up to 300 ms
of blocking sleep on the render thread.

- The lock-wait poll count is now a parameter. A start invoked on the UI
  thread passes 0, which does a single check and never sleeps; every
  other caller keeps the three-poll wait. The default is unchanged, so
  background starts behave exactly as before.

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

src/daemon/embedded_daemon.cpp

@@ -505,7 +505,7 @@ static bool isPortInUse(int port)
 #endif
 }
 
-bool EmbeddedDaemon::start(const std::string& binary_path)
+bool EmbeddedDaemon::start(const std::string& binary_path, int lockWaitPolls)
 {
     std::lock_guard<std::mutex> life(lifecycle_mutex_);
 
@@ -564,11 +564,10 @@ bool EmbeddedDaemon::start(const std::string& binary_path)
     // skip_port_check_ / -datadir override) are exempt; they run their own datadir+port.
     {
         constexpr int kDatadirLockWaitPollMs   = 100;
-        constexpr int kDatadirLockWaitMaxPolls = 3;   // ~300ms total, breaks early on exit
         bool stillRunning = false;
         if (!skip_port_check_ && override_datadir_.empty()) {
             stillRunning = isDaemonProcessRunning();
-            for (int i = 0; stillRunning && i < kDatadirLockWaitMaxPolls; ++i) {
+            for (int i = 0; stillRunning && i < lockWaitPolls; ++i) {
                 std::this_thread::sleep_for(std::chrono::milliseconds(kDatadirLockWaitPollMs));
                 stillRunning = isDaemonProcessRunning();
             }