feat: dark/light theme toggle (Catppuccin Frappé ↔ Latte)

- app/config/theme.py: palette definitions for both themes + apply_qss()
  helper used by both main.py (startup) and MainWindow (live toggle)
- resources/style_light.qss: complete Catppuccin Latte QSS
- View menu: ☀️/🌙 Switch Theme action (Ctrl+Shift+T) toggles live
- Preferences → Appearance: Color theme dropdown persists choice
- SQLHighlighter: _build_rules() reads get_palette(); update_theme()
  rebuilds rules and rehighlights all open editors on switch
- SqlCompleter: popup stylesheet reads get_palette() via
  _apply_popup_style(); update_theme() called on switch
- EditableTableModel: dirty/delete cell colors read get_palette() live
- ResultTableModel: NULL_COLOR property reads get_palette() live
- TableViewer: apply_settings() refreshes frozen-view border + repaints

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

app/ui/table_viewer.py

@@ -22,6 +22,7 @@ from PyQt6.QtGui import QColor, QBrush, QFont, QKeySequence, QShortcut
 
 from app.utils.worker import TableDataWorker
 from app.config.settings import get_settings
+from app.config.theme import get_palette
 
 
 # ─────────────────────────────────────────────────────────────────────────────
@@ -148,18 +149,20 @@ class EditableTableModel(QAbstractTableModel):
             return "" if val is None else str(val)
 
         if role == Qt.ItemDataRole.ForegroundRole:
+            p = get_palette()
             if self._is_deleted_row(r):
-                return QBrush(QColor("#e78284"))   # red — pending delete
+                return QBrush(QColor(p["delete_fg"]))
             if (r, c) in self._dirty:
-                return QBrush(QColor("#ef9f76"))   # orange — edited
+                return QBrush(QColor(p["dirty_fg"]))
             if val is None:
-                return QBrush(QColor("#737994"))   # grey — NULL
+                return QBrush(QColor(p["overlay"]))
 
         if role == Qt.ItemDataRole.BackgroundRole:
+            p = get_palette()
             if self._is_deleted_row(r):
-                return QBrush(QColor("#3a2530"))
+                return QBrush(QColor(p["delete_bg"]))
             if (r, c) in self._dirty:
-                return QBrush(QColor("#3a2f25"))
+                return QBrush(QColor(p["dirty_bg"]))
 
         return None
 
@@ -444,7 +447,7 @@ class TableViewer(QWidget):
         self._frozen_view.horizontalHeader().setSectionResizeMode(
             QHeaderView.ResizeMode.Fixed)
         self._frozen_view.setStyleSheet(
-            "QTableView { border: none; border-right: 2px solid #8caaee; }")
+            f"QTableView {{ border: none; border-right: 2px solid {get_palette()['blue']}; }}")
         self._frozen_view.hide()
 
         # Sync vertical scroll between the two views
@@ -887,6 +890,12 @@ class TableViewer(QWidget):
             )
         QApplication.clipboard().setText("\n".join(statements))
 
+    def apply_settings(self):
+        """Called by MainWindow when preferences (including theme) change."""
+        self._frozen_view.setStyleSheet(
+            f"QTableView {{ border: none; border-right: 2px solid {get_palette()['blue']}; }}")
+        self._model.layoutChanged.emit()   # repaint dirty/delete cells
+
     def _filter_by_cell(self, index):
         if not index.isValid():
             return