ipn/ipnlocal: preserve b.loginFlags in auto-login cc.Login calls

LocalBackend stores loginFlags at construction so that per-instance
properties (e.g. LoginEphemeral set by tsnet.Server.Ephemeral) persist
for the session. StartLoginInteractiveAs already merges b.loginFlags
into its cc.Login call, but the two auto-login call sites pass bare
controlclient.LoginDefault, silently dropping any stored flags.

Merge b.loginFlags at both auto-login call sites to match the existing
StartLoginInteractiveAs pattern. LoginDefault is zero so this is a
no-op when loginFlags is empty, and restores the documented behavior
when it isn't.

Fixes #15852

Signed-off-by: Scott Graham <scott.github@h4ck3r.net>

ipn/ipnlocal/state_test.go

@@ -137,6 +137,7 @@ type mockControl struct {
 	calls       []string
 	authBlocked bool
 	shutdown    chan struct{}
+	loginFlags  controlclient.LoginFlags
 
 	hi *tailcfg.Hostinfo
 }
@@ -274,6 +275,7 @@ func (cc *mockControl) Login(flags controlclient.LoginFlags) {
 	cc.mu.Lock()
 	defer cc.mu.Unlock()
 	cc.authBlocked = interact || newKeys
+	cc.loginFlags |= flags
 }
 
 func (cc *mockControl) Logout(ctx context.Context) error {