Add polygon drawing and click-to-fill tools

Introduce two new drawing modes alongside the existing brush:

- Polygon mode: left-click to place vertices connected by lines; right-click
  removes the last vertex; clicking near the first vertex (red dot) when >= 3
  points are placed closes the shape (bold cyan outline). Multiple shapes can
  coexist as canvas overlays. Cancel Current Poly discards the in-progress
  polygon; Del Shape removes the last completed shape.

- Fill mode: left-click inside any closed polygon to rasterise it onto the
  mask. Selects the innermost shape containing the click (smallest area via
  cv2.contourArea). Polygons whose centroid lies inside the target are punched
  out as holes. Each fill is a single undoable step in the mask history.

Also renames the Alpha slider label to Mask Alpha and removes the stale
Reload Saved reference from the README.

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

src/river_annotation_tool/annotator.py

@@ -119,6 +119,16 @@ class Annotator(QMainWindow):
         for b in [self.btn_prev, btn_next, btn_skip, btn_load_prev_mask]:
             row1.addWidget(b)
 
+        row_tools = QHBoxLayout()
+        for b in [
+            self.mc.btn_brush,
+            self.mc.btn_polygon,
+            self.mc.btn_fill,
+            self.mc.btn_del_shape,
+            self.mc.btn_cancel_poly,
+        ]:
+            row_tools.addWidget(b)
+
         row2 = QHBoxLayout()
         for b in [
             btn_clear,
@@ -136,7 +146,7 @@ class Annotator(QMainWindow):
         row3.addWidget(self.mc.brush_reset)
 
         row4 = QHBoxLayout()
-        row4.addWidget(QLabel("Alpha"))
+        row4.addWidget(QLabel("Mask Alpha"))
         row4.addWidget(self.mc.alpha_slider)
         row4.addWidget(self.mc.alpha_reset)
 
@@ -162,6 +172,7 @@ class Annotator(QMainWindow):
         left = QVBoxLayout()
         left.addLayout(canvas_row)
         left.addLayout(row1)
+        left.addLayout(row_tools)
         left.addLayout(row2)
         left.addLayout(row3)
         left.addLayout(row4)