feat(master): Ledger UI for Services and Agents (#18)

Restyles the master UI after the Ledger design: a sidebar with live counts,
a machine filter, a Live box and a light/dark toggle; the overview groups
services per Swarm cluster and machine with icons, search and the
Icon / Icon + text / Text switch; the agents page gets a revoke popover and
an add-agent panel with a connect step. Jobs, Job and Service pages get the
new shell and tokens only.

- fonts and Simple Icons are served by the master (no CDN)
- the overview reads agents too, to show offline banners and cluster managers
- revoking an agent also publishes the agents event
- the SSE keepalive is a named event (ping) the Live box counts
- the master passes its build version to the UI so agents on another
  version are marked old

Claude-Session: https://claude.ai/code/session_01GAZsp3P7Qto6JnhipxWUJQ

cmd/master/main.go

@@ -117,7 +117,7 @@ func run() error {
 
 	ui, err := web.New(st, agentHub, jobService, broker, web.Config{
 		AgentURL: cfg.AgentURL, MasterKey: fingerprint, AgentImage: cfg.AgentImage,
-		PollInterval: cfg.PollInterval,
+		PollInterval: cfg.PollInterval, Version: version,
 	}, logger)
 	if err != nil {
 		return err