feat(api): enhance peer session management and documentation

- Added new fields to the API for tracking connected speakers and session states across multiple nodes, including `connected_speaker_id`, `connected_speaker_label`, `session_on_speakers`, `established_on_speakers`, and `session_mismatch`.
- Implemented a new endpoint for retrieving bird protocol sessions, enhancing the agent server functionality.
- Updated the OpenAPI documentation to reflect the new fields and query parameters, improving clarity for API consumers.
- Modified the frontend to display connected speaker information and session states, providing better visibility into peer connections.
- Updated deployment documentation to clarify the configuration requirements for enabling IP forwarding on VPS.

internal/store/speaker_meta.go

@@ -124,11 +124,20 @@ func AgentSyncURL(meta SpeakerMeta) string {
 
 // AgentHealthURL returns HTTPS health URL for agent_domain.
 func AgentHealthURL(meta SpeakerMeta) string {
+	return agentHTTPSURL(meta, "/v1/agent/health")
+}
+
+// AgentBirdProtocolsURL returns HTTPS bird protocols URL for agent_domain.
+func AgentBirdProtocolsURL(meta SpeakerMeta) string {
+	return agentHTTPSURL(meta, "/v1/agent/bird/protocols")
+}
+
+func agentHTTPSURL(meta SpeakerMeta, path string) string {
 	domain := strings.TrimSpace(meta.AgentDomain)
 	if domain == "" {
 		return ""
 	}
-	return "https://" + strings.TrimSuffix(domain, "/") + "/v1/agent/health"
+	return "https://" + strings.TrimSuffix(domain, "/") + path
 }
 
 // SpeakerNeedsRemoteDispatch reports whether deploy_apply should wake this speaker via agent HTTP.