feat: shortcuts dialog, tab context menus, smart status bar, recent files

- ShortcutsDialog: grouped tree of all keyboard shortcuts (Help menu)
- Workspace tab context menu: Rename, Close, Close Others, Close to Right
- SQL query tab context menu: Rename, Duplicate, Close, Close Others
- Smart status bar: right-side connection indicator + selected cell preview
- File → Open SQL File (Ctrl+O), Save (Ctrl+S), Save As, Open Recent
- Recent files persisted to ~/.dbclient/recent_files.json (last 10)
- TableViewer.cell_selected signal wired to status bar cell preview
- EditorTab._filepath tracks the file associated with each query tab

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

app/ui/table_viewer.py

@@ -301,6 +301,7 @@ _PAGE_OPTIONS = [("50", 50), ("100", 100), ("All", 0)]
 
 class TableViewer(QWidget):
     status_message = pyqtSignal(str)
+    cell_selected  = pyqtSignal(str)    # emits display value of clicked cell
 
     def __init__(self, driver, database: str, table: str, parent=None):
         super().__init__(parent)
@@ -398,7 +399,7 @@ class TableViewer(QWidget):
         self._table_view.doubleClicked.connect(self._on_double_click)
         # Track selection to enable/disable Edit/Delete buttons.
         # Use both signals: clicked covers mouse, selectionChanged covers keyboard.
-        self._table_view.clicked.connect(lambda _: self._refresh_action_states())
+        self._table_view.clicked.connect(self._on_cell_click)
         self._table_view.selectionModel().selectionChanged.connect(
             self._on_selection_changed)
 
@@ -584,6 +585,12 @@ class TableViewer(QWidget):
 
     # ── Selection tracking ─────────────────────────────────────────────────────
 
+    def _on_cell_click(self, index: "QModelIndex") -> None:
+        self._refresh_action_states()
+        if index.isValid():
+            val = self._model.data(index, Qt.ItemDataRole.DisplayRole)
+            self.cell_selected.emit(str(val) if val is not None else "NULL")
+
     def _on_selection_changed(self, *_):
         has_sel = bool(self._table_view.selectionModel().selectedRows())
         self._edit_btn.setEnabled(has_sel)