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/canvas_state.py

@@ -192,6 +192,15 @@ class CanvasStateMixin:
         self.update()
         return True
 
+    # swap_active_slice_left_right: function definition for ScanSlicer behavior.
+    def swap_active_slice_left_right(self):
+        active_slice = self.get_active_slice()
+        if active_slice is None:
+            return False
+        active_slice.swap_left_right()
+        self.update()
+        return True
+
     # reset_active_slice_rotation: function definition for ScanSlicer behavior.
     def reset_active_slice_rotation(self):
         active_slice = self.get_active_slice()