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_unix.go

@@ -0,0 +1,20 @@
+//go:build !windows
+
+package cmd
+
+import "syscall"
+
+// isProcessRunning checks if a process with the given PID exists.
+func isProcessRunning(pid int) bool {
+	if pid <= 0 {
+		return false
+	}
+
+	err := syscall.Kill(pid, 0)
+	if err == nil {
+		return true
+	}
+
+	// EPERM means process exists but we don't have permission to signal it.
+	return err == syscall.EPERM
+}