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/preferences_dialog.py

@@ -10,6 +10,8 @@ from app.config.settings import get_settings
 _FONT_FAMILIES = ["Consolas", "JetBrains Mono", "Fira Code", "Cascadia Code", "Courier New"]
 _PAGE_SIZES    = [50, 100, 500, 1000, 0]   # 0 → All
 _PAGE_LABELS   = ["50", "100", "500", "1 000", "All"]
+_THEMES        = [("dark",  "🌙  Dark  (Catppuccin Frappé)"),
+                  ("light", "☀️  Light (Catppuccin Latte)")]
 
 
 class PreferencesDialog(QDialog):
@@ -64,6 +66,18 @@ class PreferencesDialog(QDialog):
 
         root.addWidget(results_box)
 
+        # ── Appearance ────────────────────────────────────────────────────────
+        appear_box = QGroupBox("Appearance")
+        af = QFormLayout(appear_box)
+        af.setSpacing(8)
+
+        self._theme = QComboBox()
+        for _, label in _THEMES:
+            self._theme.addItem(label)
+        af.addRow("Color theme:", self._theme)
+
+        root.addWidget(appear_box)
+
         # ── General ───────────────────────────────────────────────────────────
         general_box = QGroupBox("General")
         gf = QFormLayout(general_box)
@@ -82,7 +96,7 @@ class PreferencesDialog(QDialog):
         root.addStretch()
 
         # ── Buttons ───────────────────────────────────────────────────────────
-        note = QLabel("Font changes take effect immediately in open editors.")
+        note = QLabel("Theme and font changes take effect immediately.")
         note.setObjectName("statusLabel")
         root.addWidget(note)
 
@@ -116,6 +130,10 @@ class PreferencesDialog(QDialog):
         self._max_history.setValue(s.get("max_history", 500))
         self._auto_commit.setChecked(s.get("auto_commit", True))
 
+        theme_name = s.get("theme", "dark")
+        ti = next((i for i, (k, _) in enumerate(_THEMES) if k == theme_name), 0)
+        self._theme.setCurrentIndex(ti)
+
     def _apply(self):
         s = get_settings()
         s.set("font_family",      self._font_family.currentText())
@@ -125,6 +143,7 @@ class PreferencesDialog(QDialog):
         s.set("query_timeout",    self._timeout.value())
         s.set("max_history",      self._max_history.value())
         s.set("auto_commit",      self._auto_commit.isChecked())
+        s.set("theme",            _THEMES[self._theme.currentIndex()][0])
         s.save()
         self.settings_applied.emit()