feat: add clean command to remove temp files, logs, and cached data

Adds `unarr clean` with interactive confirmation, --dry-run, --yes,
and --all flags. Safely skips recent usenet resume files (<7 days) to
preserve download progress. Includes platform-specific PID detection
(Unix signal 0 / Windows heartbeat heuristic), CleanableBytes callback
for future heartbeat reporting, and uses shared ui.FormatBytes.

internal/agent/process_windows.go

@@ -0,0 +1,25 @@
+//go:build windows
+
+package agent
+
+import (
+	"os"
+	"time"
+)
+
+// IsProcessAlive checks if a process with the given PID is running.
+// On Windows, os.FindProcess + a zero-timeout wait is used since
+// signal 0 is not supported.
+func IsProcessAlive(pid int) bool {
+	p, err := os.FindProcess(pid)
+	if err != nil {
+		return false
+	}
+	// On Windows, FindProcess always succeeds. Use the state file's
+	// last heartbeat as a heuristic: if it's recent, the process is alive.
+	state := ReadState()
+	if state == nil || state.PID != pid {
+		return false
+	}
+	return time.Since(state.LastHeartbeat) < 2*time.Minute
+}