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/main.py

@@ -230,6 +230,14 @@ class QtScanSlicer(QMainWindow, ScanSlicerUiBuilderMixin, ScanSlicerControllerMi
         "NzRMMjEgMTYiLz48cGF0aCBkPSJNMTYgMTZoNXY1Ii8+PGNpcmNsZSBjeD0iMTIiIGN5PSIxMiIgcj0iMSIv"
         "Pjwvc3ZnPg=="
     )
+    SWAP_LR_ICON_DATA_URL = (
+        "data:image/svg+xml;base64,"
+        "PHN2ZyB4bWxucz0iaHR0cDovL3d3dy53My5vcmcvMjAwMC9zdmciIHdpZHRoPSIyNCIgaGVpZ2h0PSIyNCIg"
+        "dmlld0JveD0iMCAwIDI0IDI0IiBmaWxsPSJub25lIiBzdHJva2U9ImN1cnJlbnRDb2xvciIgc3Ryb2tlLXdp"
+        "ZHRoPSIyIiBzdHJva2UtbGluZWNhcD0icm91bmQiIHN0cm9rZS1saW5lam9pbj0icm91bmQiPjxwYXRoIGQ9"
+        "Ik04IDMgNCA3bDQgNCIvPjxwYXRoIGQ9Ik00IDdoMTYiLz48cGF0aCBkPSJtMTYgMjEgNC00LTQtNCIvPjxw"
+        "YXRoIGQ9Ik0yMCAxN0g0Ii8+PC9zdmc+"
+    )
     RESET_ROTATION_ICON_DATA_URL = (
         "data:image/svg+xml;base64,"
         "PHN2ZyB4bWxucz0iaHR0cDovL3d3dy53My5vcmcvMjAwMC9zdmciIHdpZHRoPSIyNCIgaGVpZ2h0PSIyNCIg"