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>

main.py

@@ -9,15 +9,13 @@ from PyQt6.QtGui import QFont
 
 from app.utils.logger import setup_logging, install_qt_message_handler, get_logger
 from app.main_window import MainWindow
+from app.config.theme import apply_qss
 
 log = get_logger(__name__)
 
 
 def load_stylesheet(app: QApplication) -> None:
-    style_path = os.path.join(os.path.dirname(__file__), "resources", "style.qss")
-    if os.path.exists(style_path):
-        with open(style_path, "r", encoding="utf-8") as f:
-            app.setStyleSheet(f.read())
+    apply_qss(app)   # reads theme from settings, falls back to dark
 
 
 def main():