polish: row numbers, ping indicator, frozen column, keyboard nav

- ResultsPanel: _RowNumberProxy keeps vertical header numbers sequential
  in visual order (sort-stable); both views get a 48 px wide row-number header
- MainWindow: _PingWorker pings each active connection every 30 s via
  test_connection(); status dot (green/red) shown in status bar with tooltip
- TableViewer: 📌 toggle freezes first column using dual-view overlay
  (_frozen_view child widget) with synced scroll, row heights, and resize
  event-filter; column 0 is hidden in the main view while frozen
- RowDialog: OK set as default button; first field focused on open
- ConnectionDialog: OK set as default; Enter in password field submits

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

app/ui/results_panel.py

@@ -19,6 +19,14 @@ from PyQt6.QtGui import QColor, QFont
 from app.models.result_table_model import ResultTableModel
 
 
+class _RowNumberProxy(QSortFilterProxyModel):
+    """Proxy that shows sequential visual-order row numbers in the vertical header."""
+    def headerData(self, section, orientation, role=Qt.ItemDataRole.DisplayRole):
+        if orientation == Qt.Orientation.Vertical and role == Qt.ItemDataRole.DisplayRole:
+            return str(section + 1)
+        return super().headerData(section, orientation, role)
+
+
 class ResultsPanel(QWidget):
     status_message = pyqtSignal(str)
 
@@ -86,7 +94,7 @@ class ResultsPanel(QWidget):
     def _make_table_tab(self, cols: list, rows: list) -> tuple:
         model = ResultTableModel()
         model.set_data(cols, rows)
-        proxy = QSortFilterProxyModel()
+        proxy = _RowNumberProxy()
         proxy.setSourceModel(model)
 
         table = QTableView()
@@ -99,6 +107,7 @@ class ResultsPanel(QWidget):
             QHeaderView.ResizeMode.Interactive)
         table.horizontalHeader().setStretchLastSection(True)
         table.verticalHeader().setDefaultSectionSize(24)
+        table.verticalHeader().setFixedWidth(48)
         table.setContextMenuPolicy(Qt.ContextMenuPolicy.CustomContextMenu)
         table.customContextMenuRequested.connect(
             lambda pos, t=table, p=proxy, m=model: