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>

CLAUDE.md

@@ -144,6 +144,7 @@ All four drivers have a `_where(where: dict) → (clause_str, params)` static he
 - Connection ping indicator: 30-second `_PingWorker` (QThread) pings each active connection via `test_connection()`; status dot (● green / ● red) shown in status bar right side with per-connection tooltip
 - Frozen first column in `TableViewer`: 📌 toggle button in toolbar; dual-view overlay (`_frozen_view` child of `_table_view`) with synced vertical scroll and row heights; event-filter updates geometry on resize
 - Keyboard navigation in dialogs: OK set as default button (Enter submits) in `RowDialog` and `ConnectionDialog`; focus jumps to first input field on open; Enter in password field submits `ConnectionDialog`
+- Light / dark theme toggle: `app/config/theme.py` holds Catppuccin Frappé (dark) + Latte (light) palettes; `resources/style_light.qss` is the full Latte QSS; toggle via View → Switch Theme (Ctrl+Shift+T) or Preferences → Color theme; on switch, QSS is reloaded, syntax highlighter rebuilds rules via `update_theme()`, completer popup recolors, and model dirty/NULL colors update live
 
 ### Not Yet Implemented
 **High impact:**
@@ -155,5 +156,4 @@ All four drivers have a `_where(where: dict) → (clause_str, params)` static he
 **Medium impact:**
 - Transaction panel: explicit BEGIN / COMMIT / ROLLBACK controls with in-flight indicator
 - Global schema search: search across all tables/columns/procedures in a database
-- Light theme: Catppuccin Latte or similar, toggle from Preferences
 - Schema diff: compare two database schemas and show structural differences