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/sql_editor.py

@@ -471,7 +471,7 @@ class EditorTab(QWidget):
         self._splitter.setHandleWidth(3)
 
         self._editor = CodeEditor()
-        SQLHighlighter(self._editor.document())
+        self._highlighter = SQLHighlighter(self._editor.document())
         self._completer = SqlCompleter(self._editor, parent=self)
         self._editor.set_completer(self._completer)
         if self._driver:
@@ -603,6 +603,8 @@ class EditorTab(QWidget):
 
     def apply_settings(self) -> None:
         self._editor.apply_settings()
+        self._highlighter.update_theme()
+        self._completer.update_theme()
 
 
 # ── Tabbed SQL editor container ───────────────────────────────────────────────