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/signal_client.go

@@ -103,7 +103,7 @@ func (s *SignalEventStream) Close() error {
 func (c *Client) OpenSignalStream(ctx context.Context, sessionID string) (*SignalEventStream, error) {
 	streamCtx, cancel := context.WithCancel(ctx)
 
-	url := fmt.Sprintf("%s/api/internal/stream/signal/%s/events", c.baseURL, sessionID)
+	url := fmt.Sprintf("%s/api/internal/stream/signal/%s/events", c.baseURL(), sessionID)
 	req, err := http.NewRequestWithContext(streamCtx, http.MethodGet, url, nil)
 	if err != nil {
 		cancel()