Prometheus on a node; panel under a URL prefix; open ports on migrate import

A panel without VPN (e.g. after the move) can use a Prometheus that lives on
a node, such as the one with history left on the old server (node #0). Both
directions go through the agent's WSS: the node's Prometheus scrapes
127.0.0.1:AGENT_METRICS_PORT/metrics, the agent checks its token and asks
core (metrics → metrics_res, the same exposition as /metrics, at most once
per 5 s per connection); the monitor tab reads it back through the agent,
which forwards only GET /-/ready and /api/v1/query[_range]. The panel finds
it by itself (local first, then the lowest-id node), and the install/remove
buttons work on a panel without VPN. `monitor install` on a node without its
own panel picks AGENT_METRICS_PORT, writes it to cli.env and restarts the
agent; on the old server it keeps the collected data.

The panel works behind nginx in a subfolder: relative asset and API paths,
relative directory redirects, and the client address from X-Forwarded-For /
X-Real-IP only from trusted proxies (loopback, TRUST_PROXY).

migrate import opens the moved SUB_PORT/NODE_PORT in UFW when it is active.

internal/ui/server.go

@@ -14,6 +14,7 @@ import (
 	"net/http"
 	"net/url"
 	"os"
+	"path"
 	"path/filepath"
 	"regexp"
 	"sort"
@@ -908,7 +909,9 @@ func (a *App) static(c *Ctx) bool {
 		if st, err := os.Stat(f); err == nil && !hidden {
 			if st.IsDir() {
 				if !strings.HasSuffix(c.R.URL.Path, "/") {
-					http.Redirect(c.W, c.R, c.R.URL.Path+"/", http.StatusMovedPermanently)
+					// Относительный адрес: за прокси в подпапке абсолютный увёл бы из неё.
+					c.W.Header().Set("Location", path.Base(c.R.URL.Path)+"/")
+					c.W.WriteHeader(http.StatusMovedPermanently)
 					return false
 				}
 				f = filepath.Join(f, "index.html")