feat: disable Beamline tab when active pgroup is outside token pgroups

Switching to a visitor pgroup means running a user experiment: grey out
the Beamline tab (tooltip: beamline scientists only), bring the
Experiment tab forward with Dataset path and Exp. Config. opened.
Gate applies on transitions only so manual tab choices survive the
1 Hz status ticks.

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

src/aare/gui/main_window.py

@@ -596,6 +596,9 @@ class MainWindow(QMainWindow):
         # Tracks beamline-state TRANSITIONS for the default sample-tab switch
         # (see _apply_default_sample_tab).
         self._last_beamline_state: BeamlineStateEnum | None = None
+        # None = not yet evaluated, so the first status tick always applies
+        # the gate (the GUI may start while a visitor pgroup is active).
+        self._beamline_tab_gated: bool | None = None
 
         # Wrapper for the left inset: QTabWidget ignores its own contents
         # margins for the tab bar, so the padding lives one level up. Aligns
@@ -2578,10 +2581,33 @@ class MainWindow(QMainWindow):
         else:
             self.sample_lists_tabs.setCurrentIndex(0)
 
+    def _apply_pgroup_gate(self, pgroup: str | None) -> None:
+        # Switching to a pgroup outside the token's own (admin) pgroups means
+        # running a visitor's experiment: the Beamline tab is then off-limits,
+        # and the Experiment tab comes up with Dataset path + Exp. Config.
+        # open. Applied only on gate TRANSITIONS so a manual tab choice
+        # survives the 1 Hz status ticks.
+        gated = pgroup is not None and pgroup not in (self._decoded_token.pgroups or [])
+        if gated == self._beamline_tab_gated:
+            return
+        self._beamline_tab_gated = gated
+        self.left_column_tabs.setTabEnabled(0, not gated)
+        self.left_column_tabs.setTabToolTip(
+            0, "Only accessible for beamline scientists" if gated else ""
+        )
+        if gated:
+            self.left_column_tabs.setCurrentIndex(1)
+            # persist=False: programmatic open must not overwrite the user's
+            # saved per-panel collapse choice (same rule as the session gate).
+            for banner in self.data_collection.findChildren(TitleLabel):
+                if banner.text() in ("Dataset path", "Experiment configuration"):
+                    banner.set_collapsed(False, persist=False)
+
     @Slot(DAQStatusModel)
     def update_daq_status(self, s: DAQStatusModel):
         self._latest_daq_status = s
         self._apply_session_gate(getattr(getattr(s, "session", None), "session", None))
+        self._apply_pgroup_gate(getattr(getattr(s, "session", None), "current_pgroup", None))
 
         # Default tab only on state TRANSITIONS — a manual tab choice
         # survives while the state stays put.