[client] Provision the peer identity under the config lock (review item)

Login took the authoritative update-settings and privilege decisions under
guardedConfigMu, then released it and called getConfig, which mints the peer's
identity and writes the config out. Between that read and that write, a
SetConfig holding the same lock could land a change and answer its caller —
and then be overwritten by the config the login had already read.

The window is narrow: getConfig only writes when the profile has no identity
or no file, so in practice a first login racing a settings change on the same
profile. It is also narrower than before this branch, where the write happened
inside the reader on every read that filled in a default.

Provisioning now runs where the decision it belongs to runs: at the end of
authorizeAndPrepareLogin, with the lock already held, next to
persistLoginOverrides, which writes there too. No lock is taken that was not
held before, so the documented guardedConfigMu-then-mutex order is untouched.

getConfig keeps its behaviour by calling the same extracted helper; on the
login path it now finds the identity already there and writes nothing. The
other callers are unchanged, and still provision outside any lock — a
concurrent SetConfig is not part of their flow.

Reported by cubic on the PR.

client/server/provision_identity_test.go

@@ -0,0 +1,59 @@
+package server
+
+import (
+	"os"
+	"path/filepath"
+	"testing"
+
+	"github.com/stretchr/testify/require"
+
+	"github.com/netbirdio/netbird/client/internal/profilemanager"
+)
+
+// The daemon provisions the peer's identity and persists it, because a key that
+// stayed in memory would come back different on the next start and register a
+// second peer. Provisioning is idempotent: a profile that already has an
+// identity keeps the one on disk.
+func TestProvisionProfileIdentity(t *testing.T) {
+	origDir := profilemanager.DefaultConfigPathDir
+	origPath := profilemanager.DefaultConfigPath
+	t.Cleanup(func() {
+		profilemanager.DefaultConfigPathDir = origDir
+		profilemanager.DefaultConfigPath = origPath
+	})
+
+	dir := t.TempDir()
+	profilemanager.DefaultConfigPathDir = dir
+	profilemanager.DefaultConfigPath = filepath.Join(dir, "default.json")
+
+	activeProf := &profilemanager.ActiveProfileState{ID: "default"}
+
+	t.Run("a profile with no file is provisioned and written", func(t *testing.T) {
+		_, err := os.Stat(profilemanager.DefaultConfigPath)
+		require.True(t, os.IsNotExist(err), "the fixture starts without a config file")
+
+		config, existed, err := provisionProfileIdentity(activeProf)
+		require.NoError(t, err)
+		require.False(t, existed, "the file was reported as pre-existing")
+		require.NotEmpty(t, config.PrivateKey)
+
+		stored, err := profilemanager.GetExistingConfig(profilemanager.DefaultConfigPath)
+		require.NoError(t, err, "provisioning did not write the config out")
+		require.Equal(t, config.PrivateKey, stored.PrivateKey, "the persisted identity is not the one returned")
+		require.NotEmpty(t, stored.SSHKey)
+	})
+
+	t.Run("a second call keeps the identity on disk", func(t *testing.T) {
+		before, err := profilemanager.GetExistingConfig(profilemanager.DefaultConfigPath)
+		require.NoError(t, err)
+
+		config, existed, err := provisionProfileIdentity(activeProf)
+		require.NoError(t, err)
+		require.True(t, existed)
+		require.Equal(t, before.PrivateKey, config.PrivateKey, "provisioning minted a second identity")
+
+		after, err := profilemanager.GetExistingConfig(profilemanager.DefaultConfigPath)
+		require.NoError(t, err)
+		require.Equal(t, before.PrivateKey, after.PrivateKey, "provisioning rewrote the stored identity")
+	})
+}