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_completer.py

@@ -12,6 +12,7 @@ from PyQt6.QtWidgets import QApplication, QListWidget, QListWidgetItem
 
 from app.ui.syntax_highlighter import _KEYWORDS, _TYPES, _FUNCTIONS
 from app.utils.worker import SchemaWorker
+from app.config.theme import get_palette
 
 # Combined, deduplicated keyword list used as the static completion pool.
 SQL_KEYWORDS: list[str] = list(dict.fromkeys(_KEYWORDS + _TYPES + _FUNCTIONS))
@@ -51,24 +52,28 @@ class _CompletionPopup(QListWidget):
         self.setFont(font)
         self._row_h = QFontMetrics(font).height() + 6
 
-        self.setStyleSheet("""
-            QListWidget {
-                background: #292c3c;
-                color: #c6d0f5;
-                border: 1px solid #51576d;
+        self._apply_popup_style()
+        self.setVerticalScrollBarPolicy(Qt.ScrollBarPolicy.ScrollBarAsNeeded)
+        self.setHorizontalScrollBarPolicy(Qt.ScrollBarPolicy.ScrollBarAlwaysOff)
+
+    def _apply_popup_style(self):
+        p = get_palette()
+        self.setStyleSheet(f"""
+            QListWidget {{
+                background: {p['popup_bg']};
+                color: {p['text']};
+                border: 1px solid {p['popup_border']};
                 border-radius: 4px;
                 padding: 2px;
                 outline: none;
-            }
-            QListWidget::item { padding: 2px 8px; }
-            QListWidget::item:selected {
-                background: #414559;
-                color: #c6d0f5;
-            }
-            QListWidget::item:hover { background: #363a4f; }
+            }}
+            QListWidget::item {{ padding: 2px 8px; }}
+            QListWidget::item:selected {{
+                background: {p['popup_selected']};
+                color: {p['text']};
+            }}
+            QListWidget::item:hover {{ background: {p['popup_hover']}; }}
         """)
-        self.setVerticalScrollBarPolicy(Qt.ScrollBarPolicy.ScrollBarAsNeeded)
-        self.setHorizontalScrollBarPolicy(Qt.ScrollBarPolicy.ScrollBarAlwaysOff)
 
         self.itemClicked.connect(
             lambda item: self._editor._completer._accept_completion(item.text())
@@ -142,6 +147,10 @@ class SqlCompleter(QObject):
 
     # ── Public API ────────────────────────────────────────────────────────────
 
+    def update_theme(self) -> None:
+        """Reapply popup colors after a theme change."""
+        self._popup._apply_popup_style()
+
     def set_context(self, driver, database: str) -> None:
         """Update the active driver/database and refresh schema cache."""
         self._driver   = driver