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/cmd/daemon.go

@@ -219,6 +219,7 @@ func runDaemonStart() error {
 
 	// Wire state tracking
 	d.GetActiveCount = manager.ActiveCount
+	d.GetCleanableBytes = CleanableBytes
 
 	// Wire: server-side signals -> manager actions + stream tasks
 	reporter.SetCancelHandler(func(taskID string) {