feat: dataset statistics dialog with per-video breakdown and class balance

Details button in Train dialog opens a stats view showing:
- Class totals (positive/soft/negative) with colored balance bar
- Per-video table sortable by column
- Warnings for low clip counts, class imbalance, negative-only videos

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

main.py

@@ -223,6 +223,103 @@ class ScanWorker(QThread):
                 self.error.emit(str(e))
 
 
+class DatasetStatsDialog(QDialog):
+    """Per-video dataset breakdown with class balance visualization."""
+
+    def __init__(self, video_infos: list, parent=None):
+        super().__init__(parent)
+        self.setWindowTitle("Dataset Statistics")
+        self.setMinimumSize(600, 400)
+
+        layout = QVBoxLayout(self)
+
+        # ── Totals ────────────────────────────────────────────
+        n_pos = sum(len(vi[1]) for vi in video_infos)
+        n_soft = sum(len(vi[2]) for vi in video_infos)
+        n_neg = sum(len(vi[3]) for vi in video_infos)
+        n_total = n_pos + n_soft + n_neg
+
+        totals = QLabel(
+            f"<b>{len(video_infos)}</b> videos &nbsp;|&nbsp; "
+            f"<b>{n_total}</b> total clips &nbsp;|&nbsp; "
+            f"<span style='color:#4a4'>■</span> {n_pos} positive &nbsp; "
+            f"<span style='color:#aa4'>■</span> {n_soft} soft &nbsp; "
+            f"<span style='color:#a44'>■</span> {n_neg} negative"
+        )
+        layout.addWidget(totals)
+
+        # ── Class balance bar ─────────────────────────────────
+        if n_total > 0:
+            bar = QWidget()
+            bar.setFixedHeight(20)
+            bar.setStyleSheet("background: #222;")
+
+            class _BalanceBar(QWidget):
+                def __init__(self, pos, soft, neg, total):
+                    super().__init__()
+                    self._fracs = (pos / total, soft / total, neg / total)
+                    self.setFixedHeight(20)
+
+                def paintEvent(self, _ev):
+                    p = QPainter(self)
+                    w = self.width()
+                    colors = [QColor(80, 170, 80), QColor(170, 170, 60), QColor(170, 70, 70)]
+                    x = 0
+                    for frac, col in zip(self._fracs, colors):
+                        bw = int(frac * w)
+                        if bw > 0:
+                            p.fillRect(x, 0, bw, 20, col)
+                            x += bw
+                    p.end()
+
+            balance = _BalanceBar(n_pos, n_soft, n_neg, n_total)
+            layout.addWidget(balance)
+
+        # ── Per-video table ───────────────────────────────────
+        table = QTableWidget(len(video_infos), 5)
+        table.setHorizontalHeaderLabels(["Video", "Pos", "Soft", "Neg", "Total"])
+        table.horizontalHeader().setSectionResizeMode(0, QHeaderView.ResizeMode.Stretch)
+        for c in range(1, 5):
+            table.horizontalHeader().setSectionResizeMode(c, QHeaderView.ResizeMode.ResizeToContents)
+        table.setEditTriggers(QTableWidget.EditTrigger.NoEditTriggers)
+        table.setSelectionBehavior(QTableWidget.SelectionBehavior.SelectRows)
+        table.verticalHeader().setVisible(False)
+
+        for row, (path, pos, soft, neg) in enumerate(video_infos):
+            name = os.path.basename(path)
+            table.setItem(row, 0, QTableWidgetItem(name))
+            for col, val in enumerate([len(pos), len(soft), len(neg),
+                                       len(pos) + len(soft) + len(neg)], 1):
+                item = QTableWidgetItem(str(val))
+                item.setTextAlignment(Qt.AlignmentFlag.AlignCenter)
+                table.setItem(row, col, item)
+
+        table.sortItems(1, Qt.SortOrder.DescendingOrder)
+        layout.addWidget(table)
+
+        # ── Warnings ──────────────────────────────────────────
+        warnings = []
+        if n_pos == 0:
+            warnings.append("No positive clips — export some clips first.")
+        elif n_pos < 20:
+            warnings.append(f"Only {n_pos} positive clips — aim for 20+ for decent results.")
+        # Check for videos with zero positives (only negatives)
+        neg_only = sum(1 for vi in video_infos if len(vi[1]) == 0 and len(vi[3]) > 0)
+        if neg_only:
+            warnings.append(f"{neg_only} video(s) have only negatives, no positives.")
+        # Check balance ratio
+        if n_pos > 0 and n_neg > 0 and (n_neg / n_pos > 5 or n_pos / n_neg > 5):
+            warnings.append("Class imbalance >5:1 — consider adding more of the minority class.")
+        if warnings:
+            lbl = QLabel("<br>".join(f"⚠ {w}" for w in warnings))
+            lbl.setStyleSheet("color: #cc8800;")
+            layout.addWidget(lbl)
+
+        btns = QDialogButtonBox(QDialogButtonBox.StandardButton.Close)
+        btns.rejected.connect(self.close)
+        layout.addWidget(btns)
+
+
 class TrainDialog(QDialog):
     """Dialog for configuring and launching classifier training."""
 
@@ -301,11 +398,19 @@ class TrainDialog(QDialog):
 
         layout.addLayout(form)
 
-        # Stats summary
+        # Stats summary with details button
+        stats_row = QHBoxLayout()
         self._lbl_stats = QLabel()
+        stats_row.addWidget(self._lbl_stats, 1)
+        self._btn_details = QPushButton("Details…")
+        self._btn_details.setFixedWidth(70)
+        self._btn_details.clicked.connect(self._show_details)
+        self._btn_details.setEnabled(False)
+        stats_row.addWidget(self._btn_details, 0, Qt.AlignmentFlag.AlignTop)
+        self._video_infos: list = []
         self._update_stats()
         self._cmb_positive.currentIndexChanged.connect(self._update_stats)
-        layout.addWidget(self._lbl_stats)
+        layout.addLayout(stats_row)
 
         # Buttons
         btns = QDialogButtonBox(
@@ -376,6 +481,8 @@ class TrainDialog(QDialog):
         self._lbl_video_dir.setVisible(needs_fallback)
         self._video_dir_widget.setVisible(needs_fallback)
 
+        self._video_infos = video_infos
+        self._btn_details.setEnabled(len(video_infos) > 0)
         n_videos = len(video_infos)
         n_pos = sum(len(vi[1]) for vi in video_infos)
         n_soft = sum(len(vi[2]) for vi in video_infos)
@@ -392,6 +499,11 @@ class TrainDialog(QDialog):
             lines.append("<i>Recommend at least 3 videos for decent results.</i>")
         self._lbl_stats.setText("<br>".join(lines))
 
+    def _show_details(self):
+        if self._video_infos:
+            dlg = DatasetStatsDialog(self._video_infos, parent=self)
+            dlg.exec()
+
     @property
     def positive_folder(self) -> str:
         return self._cmb_positive.currentData() or ""