feat: multi-result tabs, column statistics, and Ctrl+W close tab

- ResultsPanel rewritten: single queries show one result as before;
  scripts with multiple statements show each result in a named sub-tab
  (e.g. "Result 2 (1,234)") with the tab bar auto-shown/hidden
- EditorTab._on_script_done now calls show_script_results() instead of
  overwriting the panel on each statement
- ColumnStatsDialog: right-click any column header in TableViewer to see
  total rows, null count, distinct values, min, max, and avg (async,
  gracefully skips avg for non-numeric columns)
- TableViewer header context menu also adds "Resize to fit" shortcuts
- Ctrl+W closes the current workspace tab (View menu)

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

app/ui/table_viewer.py

@@ -399,6 +399,11 @@ class TableViewer(QWidget):
             Qt.ContextMenuPolicy.CustomContextMenu)
         self._table_view.customContextMenuRequested.connect(self._context_menu)
         self._table_view.doubleClicked.connect(self._on_double_click)
+
+        # Column header context menu
+        hdr = self._table_view.horizontalHeader()
+        hdr.setContextMenuPolicy(Qt.ContextMenuPolicy.CustomContextMenu)
+        hdr.customContextMenuRequested.connect(self._header_context_menu)
         # Track selection to enable/disable Edit/Delete buttons.
         # Use both signals: clicked covers mouse, selectionChanged covers keyboard.
         self._table_view.clicked.connect(self._on_cell_click)
@@ -629,6 +634,35 @@ class TableViewer(QWidget):
                            self._copy_selected_as_insert)
         menu.exec(self._table_view.viewport().mapToGlobal(pos))
 
+    def _header_context_menu(self, pos):
+        hdr = self._table_view.horizontalHeader()
+        col_idx = hdr.logicalIndexAt(pos)
+        if col_idx < 0 or col_idx >= len(self._model.column_names()):
+            return
+        col_name = self._model.column_names()[col_idx]
+        menu = QMenu(self)
+        menu.addAction(
+            f"📊  Statistics: {col_name}",
+            lambda: self._show_column_stats(col_name),
+        )
+        menu.addSeparator()
+        menu.addAction(
+            "⟺  Resize to fit",
+            lambda: self._table_view.resizeColumnToContents(col_idx),
+        )
+        menu.addAction(
+            "⟺  Resize all to fit",
+            self._table_view.resizeColumnsToContents,
+        )
+        menu.exec(hdr.mapToGlobal(pos))
+
+    def _show_column_stats(self, col_name: str):
+        from app.ui.column_stats_dialog import ColumnStatsDialog
+        dlg = ColumnStatsDialog(
+            self._driver, self._database, self._table, col_name, parent=self
+        )
+        dlg.exec()
+
     # ── Double-click: edit ────────────────────────────────────────────────────
 
     def _on_double_click(self, index: QModelIndex):