feat: add explicit left/right orientation with swap button

Fixes mirrored slice exports by making orientation explicit:

- PhotoSlice.swapped flag persists a user left/right swap
- swap_left_right() mirrors [TL,TR,BR,BL] -> [TR,TL,BL,BR]
- normalize_top_left() honors the flag so squarize/reset_rotation
  no longer overwrite manual orientation
- Swap L/R toolbar button wired through canvas state + controller
- Color-code left (orange) vs right (cyan) corners and edges

scanslicer/ui_builders_preview.py

@@ -123,6 +123,16 @@ class ScanSlicerUiPreviewMixin(ScanSlicerUiSharedMixin):
         )
         preview_toolbar_layout.addWidget(self.active_box_reset_rotation_button)
 
+        self.active_box_swap_button = self._create_toolbar_button(
+            preview_toolbar,
+            text="Swap L/R",
+            tooltip="Swap left/right orientation of the active box",
+            icon_data_url=self.SWAP_LR_ICON_DATA_URL,
+            callback=self.swap_active_box_left_right,
+            enabled=False,
+        )
+        preview_toolbar_layout.addWidget(self.active_box_swap_button)
+
         self.add_box_button = self._create_toolbar_button(
             preview_toolbar,
             text="Add box",