Add end-of-clips dialog and --no-skip flag

Show a modal dialog when all clips have been processed and quit cleanly.
Add --no-skip CLI flag to include already-annotated clips (default remains to skip them).

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

src/river_annotation_tool/annotator.py

@@ -6,11 +6,13 @@ import numpy as np
 from PIL import Image
 from PySide6.QtCore import QTimer
 from PySide6.QtWidgets import (
+    QApplication,
     QButtonGroup,
     QGroupBox,
     QHBoxLayout,
     QLabel,
     QMainWindow,
+    QMessageBox,
     QPushButton,
     QRadioButton,
     QVBoxLayout,
@@ -29,6 +31,7 @@ class Annotator(QMainWindow):
         config: AppConfig,
         clip: str = None,
         extras: bool = False,
+        skip_annotated: bool = True,
     ):
         super().__init__()
 
@@ -42,6 +45,7 @@ class Annotator(QMainWindow):
             clips_file=Path(config.clips_file),
             mask_filename=config.filenames.mask,
             zip_extension=config.filenames.zip_extension,
+            skip_annotated=skip_annotated,
         )
 
         self.setWindowTitle("River Annotator")
@@ -251,7 +255,16 @@ class Annotator(QMainWindow):
             self._set_answers(answers)
 
     def _advance_clip(self):
-        self._load_clip()
+        try:
+            self._load_clip()
+        except RuntimeError:
+            msg = QMessageBox(self)
+            msg.setWindowTitle("All done!")
+            msg.setText("You have reached the end of all clips.")
+            msg.setStandardButtons(QMessageBox.StandardButton.Ok)
+            msg.exec()
+            QApplication.instance().quit()
+            return
         self.frame_i = 0
         self.mc.load_clip(
             self.frames,