Add image adjustment sliders, mask alpha, Load Prev Mask, and button state colours

- Three vertical sliders (Brightness, Contrast, Gamma) to the left of the
  canvas for display-only image adjustment; all use power/linear formulae
  applied on-the-fly without touching saved data
- Alpha slider controls mask overlay transparency
- Brush size slider moved to its own row
- Each slider has a reset (↺) button restoring its default value
- Hide Mask button turns red when active; Eraser button turns orange
- Load Prev Mask button copies the saved mask from the previous clip in
  the list onto the current clip; the action is pushed onto the undo stack
  so it can be reverted with Undo
- Right survey panel narrowed (stretch factor 2 → 1)
- README Controls section updated to document all new features

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

src/river_annotation_tool/annotator.py

@@ -4,7 +4,7 @@ from pathlib import Path
 import cv2
 import numpy as np
 from PIL import Image
-from PySide6.QtCore import QTimer
+from PySide6.QtCore import Qt, QTimer
 from PySide6.QtWidgets import (
     QApplication,
     QButtonGroup,
@@ -113,9 +113,10 @@ class Annotator(QMainWindow):
         btn_undo = QPushButton("Undo")
         btn_undo10 = QPushButton("Undo×10")
         btn_redo = QPushButton("Redo")
+        btn_load_prev_mask = QPushButton("Load Prev Mask")
 
         row1 = QHBoxLayout()
-        for b in [self.btn_prev, btn_next, btn_skip]:
+        for b in [self.btn_prev, btn_next, btn_skip, btn_load_prev_mask]:
             row1.addWidget(b)
 
         row2 = QHBoxLayout()
@@ -128,13 +129,42 @@ class Annotator(QMainWindow):
             self.mc.btn_mask,
         ]:
             row2.addWidget(b)
-        row2.addWidget(QLabel("Brush"))
-        row2.addWidget(self.mc.brush_slider)
+
+        row3 = QHBoxLayout()
+        row3.addWidget(QLabel("Brush size"))
+        row3.addWidget(self.mc.brush_slider)
+        row3.addWidget(self.mc.brush_reset)
+
+        row4 = QHBoxLayout()
+        row4.addWidget(QLabel("Alpha"))
+        row4.addWidget(self.mc.alpha_slider)
+        row4.addWidget(self.mc.alpha_reset)
+
+        vert_panel = QHBoxLayout()
+        vert_panel.setContentsMargins(0, 0, 4, 0)
+        for label_text, slider, reset_btn in [
+            ("Brightness", self.mc.brightness_slider, self.mc.brightness_reset),
+            ("Contrast", self.mc.contrast_slider, self.mc.contrast_reset),
+            ("Gamma", self.mc.gamma_slider, self.mc.gamma_reset),
+        ]:
+            col = QVBoxLayout()
+            lbl = QLabel(label_text)
+            lbl.setAlignment(Qt.AlignmentFlag.AlignHCenter)
+            col.addWidget(lbl)
+            col.addWidget(slider, 1)
+            col.addWidget(reset_btn)
+            vert_panel.addLayout(col)
+
+        canvas_row = QHBoxLayout()
+        canvas_row.addLayout(vert_panel)
+        canvas_row.addWidget(self.mc.canvas, 1)
 
         left = QVBoxLayout()
-        left.addWidget(self.mc.canvas)
+        left.addLayout(canvas_row)
         left.addLayout(row1)
         left.addLayout(row2)
+        left.addLayout(row3)
+        left.addLayout(row4)
 
         left_widget = QWidget()
         left_widget.setLayout(left)
@@ -143,7 +173,7 @@ class Annotator(QMainWindow):
 
         main = QHBoxLayout()
         main.addWidget(left_widget, 3)
-        main.addWidget(right_widget, 2)
+        main.addWidget(right_widget, 1)
 
         container = QWidget()
         container.setLayout(main)
@@ -156,6 +186,7 @@ class Annotator(QMainWindow):
         btn_undo.clicked.connect(self.mc.undo)
         btn_undo10.clicked.connect(self.mc.undo10)
         btn_redo.clicked.connect(self.mc.redo)
+        btn_load_prev_mask.clicked.connect(self.load_prev_mask)
 
         if self._pending_answers:
             self._set_answers(self._pending_answers)
@@ -339,3 +370,28 @@ class Annotator(QMainWindow):
 
     def skip_clip(self):
         self._advance_clip()
+
+    def load_prev_mask(self):
+        try:
+            idx = self.selector.clips.index(self.filename)
+        except ValueError:
+            return
+        if idx == 0:
+            QMessageBox.information(
+                self, "No previous clip", "This is the first clip in the list."
+            )
+            return
+        prev_clip = self.selector.clips[idx - 1]
+        mask_path = self.out_dir / prev_clip.stem / self.cfg.filenames.mask
+        if not mask_path.exists():
+            QMessageBox.information(
+                self, "No mask found", f"No saved mask found for '{prev_clip.stem}'."
+            )
+            return
+        mask_full = np.array(Image.open(mask_path).convert("L"))
+        mask = cv2.resize(
+            (mask_full > 127).astype(np.uint8),
+            (self.dw, self.dh),
+            interpolation=cv2.INTER_NEAREST,
+        )
+        self.mc.set_mask(mask)