Add Previous button, remove Save button, warn before overwriting annotations

- Previous: saves current clip and navigates back through session history;
  disabled on the first clip, re-enabled automatically as you advance.
- Next: shows a dialog when a saved annotation already exists, letting the
  annotator choose to replace it or keep the existing save before advancing.
- Removed the standalone Save button; Next auto-saves on every advance.
- Skip already wrote nothing to disk; clarified in README.
- Refactored _advance_clip into _switch_ui_to_clip (shared with prev/next).

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

src/river_annotation_tool/annotator.py

@@ -48,14 +48,21 @@ class Annotator(QMainWindow):
             skip_annotated=skip_annotated,
         )
 
+        self.history: list[Path] = []
+        self.history_pos: int = -1
+
         self.setWindowTitle("River Annotator")
         self._load_clip(specific=clip)
+        self._history_push()
         self._init_ui()
         self._init_timer()
 
     # ── clip loading ───────────────────────────────────────────────
-    def _load_clip(self, specific: str = None):
-        self.filename = self.selector.next(specific=specific)
+    def _load_clip(self, specific: str = None, path: Path = None):
+        if path is not None:
+            self.filename = path
+        else:
+            self.filename = self.selector.next(specific=specific)
         self.frames, self.fps, self.dh, self.dw, self.h, self.w = load_frames(
             self.filename,
             self.cfg.max_frames,
@@ -66,6 +73,11 @@ class Annotator(QMainWindow):
         )
         self._pending_answers = self._read_saved_answers()
 
+    def _history_push(self):
+        del self.history[self.history_pos + 1 :]
+        self.history.append(self.filename)
+        self.history_pos = len(self.history) - 1
+
     def _read_saved_mask(self):
         mask_path = self.out_dir / self.filename.stem / self.cfg.filenames.mask
         if not mask_path.exists():
@@ -93,7 +105,8 @@ class Annotator(QMainWindow):
         self.q_widgets = {}
         question_panel = self._build_question_panel()
 
-        btn_save = QPushButton("Save")
+        self.btn_prev = QPushButton("Previous")
+        self.btn_prev.setEnabled(False)
         btn_next = QPushButton("Next")
         btn_skip = QPushButton("Skip")
         btn_clear = QPushButton("Clear")
@@ -101,7 +114,7 @@ class Annotator(QMainWindow):
         btn_reload = QPushButton("Reload Saved")
 
         row1 = QHBoxLayout()
-        for b in [btn_save, btn_next, btn_skip]:
+        for b in [self.btn_prev, btn_next, btn_skip]:
             row1.addWidget(b)
 
         row2 = QHBoxLayout()
@@ -128,7 +141,7 @@ class Annotator(QMainWindow):
         container.setLayout(main)
         self.setCentralWidget(container)
 
-        btn_save.clicked.connect(self.save)
+        self.btn_prev.clicked.connect(self.prev_clip)
         btn_next.clicked.connect(self.next_clip)
         btn_skip.clicked.connect(self.skip_clip)
         btn_clear.clicked.connect(self.mc.clear)
@@ -254,17 +267,7 @@ class Annotator(QMainWindow):
         if answers:
             self._set_answers(answers)
 
-    def _advance_clip(self):
-        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
+    def _switch_ui_to_clip(self):
         self.frame_i = 0
         self.mc.load_clip(
             self.frames,
@@ -276,10 +279,63 @@ class Annotator(QMainWindow):
         if self._pending_answers:
             self._set_answers(self._pending_answers)
             self._pending_answers = None
+        self.btn_prev.setEnabled(self.history_pos > 0)
+
+    def _advance_clip(self):
+        if self.history_pos < len(self.history) - 1:
+            self.history_pos += 1
+            self._load_clip(path=self.history[self.history_pos])
+            self._switch_ui_to_clip()
+            return
+        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._history_push()
+        self._switch_ui_to_clip()
+
+    def prev_clip(self):
+        if self.history_pos <= 0:
+            return
+        self.save()
+        self.history_pos -= 1
+        self._load_clip(path=self.history[self.history_pos])
+        self._switch_ui_to_clip()
 
     def next_clip(self):
-        self.save()
-        self._advance_clip()
+        mask_path = self.out_dir / self.filename.stem / self.cfg.filenames.mask
+        if mask_path.exists():
+            msg = QMessageBox(self)
+            msg.setWindowTitle("Existing annotation found")
+            msg.setText(
+                f"'{self.filename.stem}' already has a saved annotation.\n"
+                "Replace it with your current work, or keep the existing save?"
+            )
+            btn_replace = msg.addButton(
+                "Replace & Continue", QMessageBox.ButtonRole.AcceptRole
+            )
+            btn_keep = msg.addButton(
+                "Keep Existing & Continue", QMessageBox.ButtonRole.AcceptRole
+            )
+            msg.addButton("Cancel", QMessageBox.ButtonRole.RejectRole)
+            msg.setDefaultButton(btn_replace)
+            msg.exec()
+            clicked = msg.clickedButton()
+            if clicked == btn_replace:
+                self.save()
+                self._advance_clip()
+            elif clicked == btn_keep:
+                self._advance_clip()
+            # Cancel: do nothing
+        else:
+            self.save()
+            self._advance_clip()
 
     def skip_clip(self):
         self._advance_clip()