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/models/result_table_model.py

@@ -7,11 +7,15 @@ from PyQt6.QtCore import (
 )
 from PyQt6.QtGui import QColor, QFont
 
+from app.config.theme import get_palette
+
 
 class ResultTableModel(QAbstractTableModel):
     """Immutable result-set model — replaces data via set_data()."""
 
-    NULL_COLOR  = QColor("#737994")   # muted grey for NULL
+    @property
+    def NULL_COLOR(self):
+        return QColor(get_palette()["overlay"])
     NUM_ALIGN   = Qt.AlignmentFlag.AlignRight | Qt.AlignmentFlag.AlignVCenter
     TEXT_ALIGN  = Qt.AlignmentFlag.AlignLeft  | Qt.AlignmentFlag.AlignVCenter