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_render.py

@@ -28,14 +28,22 @@ class CanvasRenderMixin:
                 b = points[(i + 1) % 4]
                 if i == 0:
                     color = QColor("red")
+                elif i == 1:
+                    color = QColor("green")
+                elif i == 2:
+                    color = QColor("orange")
                 else:
                     color = QColor("blue") if s.locked else QColor("lightgreen")
                 painter.setPen(QPen(color, 3))
                 painter.drawLine(a, b)
 
-            painter.setPen(QPen(QColor("blue"), 2))
-            painter.setBrush(QColor(128, 128, 128, 170))
-            for p in points:
+            # Corners are color-coded: left = orange, right = cyan.
+            # Edge 0 (top, red) runs TL->TR, so corners 0 and 3 are the left
+            # side and corners 1 and 2 are the right side.
+            corner_colors = (QColor("orange"), QColor("cyan"), QColor("cyan"), QColor("orange"))
+            for p, color in zip(points, corner_colors):
+                painter.setPen(QPen(QColor("blue"), 2))
+                painter.setBrush(color)
                 painter.drawRect(QRectF(p.x() - 5, p.y() - 5, 10, 10))
 
             center = np.mean(s.bbox, axis=0)