Move the panel to a server without VPN; servers for all master keys at once

The «Перенос» tab moves a panel + VPN server to a panel-only server without
users noticing: the new panel issues a one-time awgmove:// code, the old one
sends its ui.db, subscription keys and hub certificate encrypted with it, and
becomes node #0 of the new panel. Server id 0 is kept, so config servers[] is
byte-identical. The old server relays its former ports (awg-relay, PROXY v1
header trusted only from RELAY_FROM), so sen:// links, nodes and the API keep
working. The same is available from the CLI: migrate export/import/cancel,
rehome.

Nodes now learn the panel's address: the hub sends it in ready, the agent
stores it in CORE_ENDPOINT, dials it first and confirms with core_ok. The
new panel shows when the old server can be turned off: devices that got the
new address, nodes that confirmed it, master keys never opened.

«Серверы для всех» on the master keys tab adds or removes servers on every
live key in one go, with a dry run that warns about API keys that would lose
sight of some keys, and sets the servers new keys get.

internal/envfile/envfile.go

@@ -0,0 +1,43 @@
+// Package envfile — правка cli.env (KEY=value построчно): CLI и панель пишут в него одинаково.
+package envfile
+
+import (
+	"os"
+	"regexp"
+	"strings"
+)
+
+// Update — как String.replace без g: первое KEY=…, а нет его — строка в конец.
+func Update(content, key, value string) string {
+	re := regexp.MustCompile(`(?m)^` + regexp.QuoteMeta(key) + `=.*$`)
+	if loc := re.FindStringIndex(content); loc != nil {
+		return content[:loc[0]] + key + "=" + value + content[loc[1]:]
+	}
+	return strings.TrimRightFunc(content, func(r rune) bool { return r == ' ' || r == '\n' || r == '\t' || r == '\r' }) + "\n" + key + "=" + value + "\n"
+}
+
+func Remove(content, key string) string {
+	var kept []string
+	for _, l := range strings.Split(content, "\n") {
+		if !strings.HasPrefix(l, key+"=") {
+			kept = append(kept, l)
+		}
+	}
+	return strings.Join(kept, "\n")
+}
+
+// Set — задать set и убрать remove в файле. Новый файл — 0600 (в нём пароль панели).
+func Set(file string, set [][2]string, remove ...string) error {
+	raw, err := os.ReadFile(file)
+	if err != nil && !os.IsNotExist(err) {
+		return err
+	}
+	content := string(raw)
+	for _, kv := range set {
+		content = Update(content, kv[0], kv[1])
+	}
+	for _, k := range remove {
+		content = Remove(content, k)
+	}
+	return os.WriteFile(file, []byte(content), 0o600)
+}