feat(agent): add mirror failover, agent client refactor, status 401 detection

- Mirror pool with health tracking and exponential backoff for failed hosts
- Agent client routes requests through mirror pool with retry semantics
- New `unarr mirrors` command to inspect mirror state and force failover
- `unarr status` now detects 401 from /agent/register and suggests `unarr login`
  instead of the generic "Could not fetch account info" message
- Config supports multiple ScanPaths for upcoming multi-path library scan
- Draft plan for bidirectional library sync (CLI ↔ Web) under Docs/plans/

internal/cmd/root.go

@@ -108,6 +108,8 @@ Source:         https://github.com/torrentclaw/unarr`,
 	probeHWAccelCmd.GroupID = "system"
 	cleanCmd := newCleanCmd()
 	cleanCmd.GroupID = "system"
+	mirrorsCmd := newMirrorsCmd()
+	mirrorsCmd.GroupID = "system"
 	selfUpdateCmd := newSelfUpdateCmd()
 	selfUpdateCmd.GroupID = "system"
 	versionCmd := newVersionCmd()
@@ -144,6 +146,7 @@ Source:         https://github.com/torrentclaw/unarr`,
 		doctorCmd,
 		probeHWAccelCmd,
 		cleanCmd,
+		mirrorsCmd,
 		selfUpdateCmd,
 		versionCmd,
 		completionCmd,