fix: Windows build support with platform-specific process/signal handling

Separate platform-dependent code into build-tagged files:
- process_unix.go / process_windows.go: isProcessRunning() implementation
- signals_unix.go / signals_windows.go: daemon signal handling (Windows lacks SIGUSR1)

Windows implementation uses windows.OpenProcess with PROCESS_QUERY_LIMITED_INFORMATION
and checks exit code against STILL_ACTIVE (259).

Original-PR: #447
Co-Authored-By: Johann Dirry <johann.dirry@microsea.at>

internal/cmd/process_windows.go

@@ -0,0 +1,27 @@
+//go:build windows
+
+package cmd
+
+import "golang.org/x/sys/windows"
+
+const processStillActive = 259
+
+// isProcessRunning checks if a process with the given PID exists.
+func isProcessRunning(pid int) bool {
+	if pid <= 0 {
+		return false
+	}
+
+	handle, err := windows.OpenProcess(windows.PROCESS_QUERY_LIMITED_INFORMATION, false, uint32(pid))
+	if err != nil {
+		return false
+	}
+	defer windows.CloseHandle(handle)
+
+	var exitCode uint32
+	if err := windows.GetExitCodeProcess(handle, &exitCode); err != nil {
+		return false
+	}
+
+	return exitCode == processStillActive
+}