Fix polygon canvas unzooming when mouse leaves axes

Disable matplotlib autoscale after imshow so polygon plot() calls
(rubber-band line, vertices) can't expand the view limits. Also reset
xlim/ylim explicitly in load_clip for clips with different resolutions.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/river_annotation_tool/mask_canvas.py

@@ -51,6 +51,7 @@ class MaskCanvas:
             (0, 0), radius=5, fill=False, color="white", linewidth=1.5, visible=False
         )
         self.ax.add_patch(self.brush_circle)
+        self.ax.autoscale(False)  # prevent polygon plot() calls from expanding the view
 
     def _build_controls(self):
         self.btn_erase = QPushButton("Eraser")
@@ -137,6 +138,8 @@ class MaskCanvas:
         self._current_frame = frames[0]
         self._clear_poly_state()
         self.img_artist.set_data(self._apply_image_adjustments(frames[0]))
+        self.ax.set_xlim(-0.5, dw - 0.5)
+        self.ax.set_ylim(dh - 0.5, -0.5)
         self.set_title(title)
         self.redraw()