feat: sample camera starts in scale-to-fit

_autoscale now defaults on, so the view fits from the first layout
(the constructor's placeholder already carries the camera size and
resizeEvent refits on every resize). update_pixmap additionally
refits when the frame size changes - the real stream resolution can
differ from the placeholder, and only a view resize refit before.
Right-click 'Scale to fit' still toggles back to 1:1.

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

src/aare/gui/widgets/camera_image.py

@@ -133,7 +133,9 @@ class SampleCameraImageLabel(QGraphicsView):
 
         self._geom = geom
         self._bookmarks: SmargonBookmarkList = SmargonBookmarkList()
-        self._autoscale = False
+        # Fit-to-view from the first frame; the right-click "Scale to fit"
+        # toggle can still switch back to 1:1.
+        self._autoscale = True
         self._show_coords = False
         self._helical_start = SmargonCoordinate()
         self._helical_end = SmargonCoordinate()
@@ -776,12 +778,18 @@ class SampleCameraImageLabel(QGraphicsView):
 
     @Slot(QPixmap)
     def update_pixmap(self, pixmap: QPixmap):
+        size_changed = self.pixmap_item is None or self.pixmap_item.pixmap().size() != pixmap.size()
         if self.pixmap_item is not None:  # Ensure pixmap_item exists
             self.pixmap_item.setPixmap(pixmap)  # Update the pixmap in the item
         else:
             # If no pixmap item exists (rare case), create one
             self.pixmap_item = QGraphicsPixmapItem(pixmap)
             self.scene.addItem(self.pixmap_item)
+        if size_changed and self._autoscale:
+            # Refit when the frame size differs from what was fitted (real
+            # stream resolution vs the 2000x2000 startup placeholder, or a
+            # camera source switch) — resizeEvent only refits on view resize.
+            self._scaling()
         self.viewport().update()  # Request an update to redraw the view
 
     @Slot(DAQStatusModel)

tests/unit/gui/test_camera_image.py

@@ -209,3 +209,19 @@ def test_alt_wheel_axis_swap_still_changes_exposure(camera):
     n = len(sent)
     camera.wheelEvent(_wheel(camera))  # zero delta: ignored
     assert len(sent) == n
+
+
+def test_autoscale_fits_from_the_first_frame(camera):
+    from PySide6.QtGui import QPixmap
+
+    # Fit-to-view is the default; a frame whose size differs from the fitted
+    # one (here: the 2000x2000 startup placeholder) must refit immediately,
+    # not wait for the next view resize.
+    assert camera._autoscale
+    camera.update_pixmap(QPixmap(4000, 4000))
+    assert camera.transform().m11() < 1.0
+
+    # The right-click toggle still restores 1:1.
+    camera._autoscale = False
+    camera._scaling()
+    assert camera.transform().isIdentity()