feat: cluster reliability (leader failover, crash rejoin, key exchange) + auth/users + canvas/forwards enhancements + comprehensive README + API docs

- Cluster: forwardToNext offline detection (leader+non-leader), WatchLeader 1s heartbeat fallback, 409 for standalone nodes, Node.NodeKey key exchange via token ring, ClusterPeers persistence + auto-rejoin, Forward delegates to forwardToNext (bugfix)
- Auth: Basic Auth (flag-creds fast path) + bcrypt users (admin/viewer) + Bearer API keys (read/write/admin scope)
- Frontend: UsersView (accounts+API keys), ClusterView (ring/nodeKey/tasks/topology/log), StatusView (group management, per-proxy status), CanvasView (edge toggle/group), PortEdge (disabled/group labels)
- API: handlers split (canvas/forwards/users/logs), canvas export/import, forwards group start/stop/assign/delete, cluster endpoints
- Docs: comprehensive README rewrite (all flags/APIs/auth/cluster), docs/cluster-api.md (cluster management API reference)
- Deploy: run-cluster.sh now 4-node ring + 1 isolated standalone, test-forward.sh updated for 4 nodes
- Removed plan.md (design notes consolidated into README + API docs)

go.mod

@@ -2,7 +2,10 @@ module webui4frpc
 
 go 1.25.0
 
-require modernc.org/sqlite v1.55.0
+require (
+	golang.org/x/crypto v0.55.0
+	modernc.org/sqlite v1.55.0
+)
 
 require (
 	github.com/dustin/go-humanize v1.0.1 // indirect
@@ -10,7 +13,7 @@ require (
 	github.com/mattn/go-isatty v0.0.20 // indirect
 	github.com/ncruces/go-strftime v1.0.0 // indirect
 	github.com/remyoudompheng/bigfft v0.0.0-20230129092748-24d4a6f8daec // indirect
-	golang.org/x/sys v0.46.0 // indirect
+	golang.org/x/sys v0.47.0 // indirect
 	modernc.org/libc v1.74.1 // indirect
 	modernc.org/mathutil v1.7.1 // indirect
 	modernc.org/memory v1.11.0 // indirect