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/agent/client_test.go

@@ -498,8 +498,8 @@ func TestClient_SlowServer_Timeout(t *testing.T) {
 
 	// Crear cliente con timeout muy corto
 	c := &Client{
-		baseURL: srv.URL,
-		apiKey:  "test-key",
+		pool:   NewMirrorPool(srv.URL, nil),
+		apiKey: "test-key",
 		httpClient: &http.Client{
 			Timeout: 50 * time.Millisecond,
 		},