Implement column menus for ResultsGrid with comprehensive feature set

## Summary
Add interactive column menus to the ResultsGrid component with 5 feature categories:
1. **Actions** - Hide/show columns, pin/unpin to left/right
2. **Statistics** - Calculate and display column stats (count, null count, distinct, min/max/avg)
3. **Filters** - Show top 10 values and data quality indicators
4. **Transform** - SQL transformations (UPPER, LOWER, TRIM, CAST, TRUNCATE, SUBSTRING)
5. **Export** - Copy to clipboard or export as CSV

## Architecture
- **SQL Transformation Utility** (src/utils/sqlTransformations.ts): Regex-based transformation logic
  - Handles simple columns, table prefixes, backticks, and aliases
  - Safe column wrapping with SQL functions
  - Client-side statistics calculation

- **ColumnMenu Component** (src/components/results/ColumnMenu.tsx): Custom menu with 5 tabs
  - Statistics calculation from displayed data
  - Column visibility and pinning controls
  - Top 10 values preview for quick data exploration
  - Text transformations and type casting options

- **CustomHeader Component** (src/components/results/CustomHeader.tsx): AG Grid header integration
  - Menu button in column headers using Popover
  - Connects to ColumnMenu for user interactions

- **ResultsGrid Updates** (src/components/results/ResultsGrid.tsx):
  - Column state persistence to localStorage
  - New toolbar buttons for column and sort management
  - Modal dialogs for managing column visibility and sort order
  - Event listeners for saving state on column changes

- **SqlLabPage Handler** (src/pages/SqlLabPage.tsx):
  - handleTransformColumn callback that shows preview modal
  - Transformation confirmation before modifying SQL
  - Auto-executes query after transformation

## Features Implemented
✅ Column Actions (hide/show, pin/unpin)
✅ Column Statistics (count, nulls, distinct, min/max/avg, percentages)
✅ Data Quality Indicators (non-null %, uniqueness %)
✅ Quick Data Preview (top 10 values)
✅ SQL Transformations (UPPER, LOWER, TRIM, CAST, TRUNCATE, SUBSTRING)
✅ Export Functions (copy to clipboard, export as CSV)
✅ Column State Persistence (localStorage)
✅ Sort Management (view current sorts, clear all)
✅ Column Management (show/hide all columns)

## Technical Details
- Regex-based SQL transformation instead of parser library (avoids 500KB+ dependency)
- Statistics computed on displayed/paginated data (not full dataset)
- AG Grid API integration for column visibility and pinning
- localStorage for persisting column state across sessions
- TypeScript interfaces for type safety
- Ant Design components for consistent UI

Author: cgivre