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/root.go

@@ -100,6 +100,8 @@ Source:         https://github.com/torrentclaw/torrentclaw-cli`,
 	statsCmd.GroupID = "system"
 	doctorCmd := newDoctorCmd()
 	doctorCmd.GroupID = "system"
+	cleanCmd := newCleanCmd()
+	cleanCmd.GroupID = "system"
 	selfUpdateCmd := newSelfUpdateCmd()
 	selfUpdateCmd.GroupID = "system"
 	versionCmd := newVersionCmd()
@@ -128,6 +130,7 @@ Source:         https://github.com/torrentclaw/torrentclaw-cli`,
 		// System & Diagnostics
 		statsCmd,
 		doctorCmd,
+		cleanCmd,
 		selfUpdateCmd,
 		versionCmd,
 		completionCmd,