Fix cross-platform builds: process cleanup and disk space checking

- Add platform-specific implementations for Windows, BSD systems
- Create platform-specific disk space checking with proper syscalls
- Add Windows process cleanup using tasklist/taskkill
- Add BSD-specific Statfs_t field handling (F_blocks, F_bavail, F_bsize)
- Support 9/10 target platforms (Linux, Windows, macOS, FreeBSD, OpenBSD)
- Process cleanup now works on all Unix-like systems and Windows
- Phase 2 TUI improvements compatible across platforms

internal/checks/types.go

@@ -0,0 +1,29 @@
+package checks
+
+import "fmt"
+
+// DiskSpaceCheck represents disk space information
+type DiskSpaceCheck struct {
+	Path           string
+	TotalBytes     uint64
+	AvailableBytes uint64
+	UsedBytes      uint64
+	UsedPercent    float64
+	Sufficient     bool
+	Warning        bool
+	Critical       bool
+}
+
+// formatBytes formats bytes to human-readable format
+func formatBytes(bytes uint64) string {
+	const unit = 1024
+	if bytes < unit {
+		return fmt.Sprintf("%d B", bytes)
+	}
+	div, exp := uint64(unit), 0
+	for n := bytes / unit; n >= unit; n /= unit {
+		div *= unit
+		exp++
+	}
+	return fmt.Sprintf("%.1f %ciB", float64(bytes)/float64(div), "KMGTPE"[exp])
+}