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/daemon.go

@@ -39,7 +39,8 @@ type Daemon struct {
 	heartbeatFailures int
 
 	// Callbacks for state tracking (set by cmd/daemon.go)
-	GetActiveCount func() int
+	GetActiveCount    func() int
+	GetCleanableBytes func() int64
 
 	// Exposed tickers for hot-reload
 	PollTicker      *time.Ticker