feat(stalker): protocol-correct auth lifecycle (#1354)

.changes/stalker-auth-lifecycle.md

@@ -0,0 +1,10 @@
+---
+type: feature
+area: stalker
+---
+
+Stalker portals that ask for a login and password now work: the import dialog
+gained username/password fields and the app completes the portal's do_auth
+step. When a portal refuses access, its own explanation is shown instead of a
+generic error, saved sessions resume without re-authenticating, and the
+keep-alive ping follows the portal's cadence.