fix: explain gated Beamline tab clicks; drop login state POST

Two changes folded together because they touch the same status path:

- Revert the login-default state POST: a state transition can move
  motors, and merely opening the GUI must never move hardware. The GUI
  now adopts whatever state the server reports.
- A click on the pgroup-gated (disabled) Beamline tab used to be eaten
  silently by the tab bar. An event filter now pops the explanation:
  visitor-pgroup wording for staff, the staff-only message for users
  (same pattern as the Auxiliary-puck tab).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/aare/gui/main_window.py

@@ -173,7 +173,6 @@ class MainWindow(QMainWindow):
     _default_dock_split_done: bool = False
     _pre_watch_dock_state: QByteArray | None = None
     _pre_watch_visibility: list[tuple[QWidget, bool]] | None = None
-    _login_default_state_sent: bool = False
 
     def __init__(
         self,
@@ -404,6 +403,10 @@ class MainWindow(QMainWindow):
 
         self.left_column_tabs.addTab(beamline_page, "Beamline")
         self.left_column_tabs.addTab(experiment_page, "Experiment")
+        # A click on the pgroup-gated (disabled) Beamline tab is otherwise
+        # eaten silently by the tab bar — the filter pops the explanation
+        # instead (same pattern as the Auxiliary-puck tab).
+        self.left_column_tabs.tabBar().installEventFilter(self)
 
         # Only the visible page counts toward the height — same trick as the
         # content stack below, else the taller page pads the other tab.
@@ -1424,6 +1427,20 @@ class MainWindow(QMainWindow):
             "to configure the beamline.",
         )
 
+    def _show_beamline_tab_gated_popup(self) -> None:
+        # The tab is only ever disabled by the pgroup gate; staff hitting it
+        # are running a visitor's pgroup, users simply lack the rights.
+        if self._decoded_token.staff:
+            QMessageBox.information(
+                self,
+                "Beamline tab disabled",
+                "The active pgroup is not one of yours, so the Beamline tab "
+                "is disabled while running a visitor's experiment. Switch "
+                "back to your own pgroup to configure the beamline.",
+            )
+        else:
+            self._show_beamline_staff_only_popup()
+
     @Slot()
     def _raise_reference_tools(self) -> None:
         if not self._decoded_token.staff:
@@ -2643,17 +2660,9 @@ class MainWindow(QMainWindow):
         self._apply_session_gate(getattr(getattr(s, "session", None), "session", None))
         self._apply_pgroup_gate(getattr(getattr(s, "session", None), "current_pgroup", None))
 
-        # Login default: staff start in BeamLocation, users in SampleAlignment.
-        # One-shot on the first status tick where this GUI owns the session and
-        # the beamline is idle — a busy beamline or watch-only GUI must never
-        # have its state yanked by someone merely logging in.
-        if not self._login_default_state_sent and self._session_operations_enabled and not s.busy:
-            self._login_default_state_sent = True
-            if self._decoded_token.staff:
-                if s.state != BeamlineStateEnum.BeamLocation:
-                    self.daq.beam_location()
-            elif s.state != BeamlineStateEnum.SampleAlignment:
-                self.daq.sample_alignment()
+        # No login-default state: the GUI adopts whatever state the server
+        # reports and never posts a transition on startup — a state POST can
+        # move motors, and merely logging in must not move hardware.
 
         # Default tab only on state TRANSITIONS — a manual tab choice
         # survives while the state stays put.
@@ -3225,6 +3234,18 @@ class MainWindow(QMainWindow):
         if event.type() == QEvent.Type.MouseButtonPress and obj in self._locked_beamline_banners:
             self._show_beamline_staff_only_popup()
             return True
+        # Click on the pgroup-gated (disabled) Beamline tab: tabAt() is
+        # geometric, so it still sees the disabled tab under the cursor.
+        left_tab_bar = self.left_column_tabs.tabBar()
+        if (
+            event.type() == QEvent.Type.MouseButtonPress
+            and left_tab_bar is not None
+            and obj is left_tab_bar
+            and left_tab_bar.tabAt(event.position().toPoint()) == 0
+            and not self.left_column_tabs.isTabEnabled(0)
+        ):
+            self._show_beamline_tab_gated_popup()
+            return True
         return super().eventFilter(obj, event)
 
     def _start_remote_close_countdown(