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/main_window.py

@@ -15,7 +15,7 @@ from PyQt6.QtWidgets import (
     QTabWidget, QStatusBar, QLabel, QMessageBox, QDockWidget,
     QPushButton, QApplication, QMenu, QInputDialog, QFileDialog,
 )
-from PyQt6.QtCore import Qt, QTimer
+from PyQt6.QtCore import Qt, QTimer, QThread, pyqtSignal
 from PyQt6.QtGui import QAction, QColor, QKeySequence
 
 from app.ui.schema_browser    import SchemaBrowser
@@ -41,6 +41,23 @@ from app.utils.logger import get_logger
 _log = get_logger(__name__)
 
 
+class _PingWorker(QThread):
+    """Background thread that pings one connection and emits the result."""
+    pinged = pyqtSignal(str, bool)   # (profile_id, ok)
+
+    def __init__(self, pid: str, driver, parent=None):
+        super().__init__(parent)
+        self._pid    = pid
+        self._driver = driver
+
+    def run(self):
+        try:
+            ok, _ = self._driver.test_connection()
+            self.pinged.emit(self._pid, ok)
+        except Exception:
+            self.pinged.emit(self._pid, False)
+
+
 class MainWindow(QMainWindow):
 
     def __init__(self):
@@ -53,11 +70,19 @@ class MainWindow(QMainWindow):
         self._active_drivers: dict = {}
         # profile_id → ConnectionProfile  (all loaded profiles, connected or not)
         self._all_profiles:   dict = {}
+        # ping state
+        self._ping_results: dict = {}   # profile_id → bool
+        self._ping_workers: list = []   # keep QThread refs alive
 
         self._build_ui()
         self._build_menus()
         self._build_status_bar()
 
+        # Periodic connection heartbeat (every 30 s)
+        self._ping_timer = QTimer(self)
+        self._ping_timer.timeout.connect(self._ping_all_connections)
+        self._ping_timer.start(30_000)
+
         # Load saved profiles after the window is shown
         QTimer.singleShot(0, self._load_saved_profiles)
 
@@ -234,6 +259,12 @@ class MainWindow(QMainWindow):
         self._conn_lbl.setMinimumWidth(160)
         sb.addPermanentWidget(self._conn_lbl)
 
+        self._ping_lbl = QLabel("")
+        self._ping_lbl.setFixedWidth(16)
+        self._ping_lbl.setAlignment(Qt.AlignmentFlag.AlignCenter)
+        self._ping_lbl.setToolTip("Connection heartbeat")
+        sb.addPermanentWidget(self._ping_lbl)
+
     def _set_status(self, msg: str):
         self._status_lbl.setText(msg)
 
@@ -404,6 +435,7 @@ class MainWindow(QMainWindow):
         else:
             self._schema_browser.remove_connection(profile_id, keep_saved=False)
         self._all_profiles.pop(profile_id, None)
+        self._ping_results.pop(profile_id, None)
         delete_profile(profile_id)
         self._set_status(f"Connection '{profile.name}' deleted.")
 
@@ -484,6 +516,44 @@ class MainWindow(QMainWindow):
             preview = value[:60] + ("…" if len(value) > 60 else "")
             self._cell_lbl.setText(f"  {preview}")
 
+    # ── Connection heartbeat ──────────────────────────────────────────────────
+
+    def _ping_all_connections(self):
+        # Drop finished workers
+        self._ping_workers = [w for w in self._ping_workers if w.isRunning()]
+        if not self._active_drivers:
+            self._ping_lbl.setText("")
+            return
+        for pid, driver in list(self._active_drivers.items()):
+            # Skip if a ping for this connection is already in flight
+            if any(getattr(w, "_pid", None) == pid for w in self._ping_workers):
+                continue
+            worker = _PingWorker(pid, driver)
+            worker.pinged.connect(self._on_ping_result)
+            worker.start()
+            self._ping_workers.append(worker)
+
+    def _on_ping_result(self, pid: str, ok: bool):
+        if pid not in self._active_drivers:
+            return
+        self._ping_results[pid] = ok
+        all_ok = all(self._ping_results.get(p, True) for p in self._active_drivers)
+        color = "#a6d189" if all_ok else "#e78284"
+        self._ping_lbl.setText("●")
+        self._ping_lbl.setStyleSheet(f"color: {color}; font-size: 11pt;")
+        lines = []
+        for p_id in self._active_drivers:
+            profile = self._all_profiles.get(p_id)
+            name = profile.name if profile else p_id
+            icon = "✅" if self._ping_results.get(p_id, True) else "❌"
+            lines.append(f"{icon} {name}")
+        self._ping_lbl.setToolTip("Connections:\n" + "\n".join(lines))
+        if not ok:
+            profile = self._all_profiles.get(pid)
+            name = profile.name if profile else pid
+            _log.warning("Heartbeat failed for '%s'", name)
+            self._set_status(f"⚠️  Connection heartbeat failed: {name}")
+
     # ── File open / save ──────────────────────────────────────────────────────
 
     def _open_sql_file(self, filepath: str = None):