feat: implement banner plugin

[prosdev]

Implements Issue #7: Banner Plugin

Changes

• Add bannerPlugin for rendering banner experiences
• Creates fixed position banner at top or bottom of page
• Supports dismissable close button with smooth interactions
• Supports configurable position (top/bottom), dismissable, and zIndex
• Auto-shows banners on experiences:evaluated event
• Emits experiences:shown and experiences:dismissed events
• Cleans up properly on sdk:destroy event
• Full width responsive banner with flexbox layout
• Inline styles for zero CSS dependencies

Testing

• 24 comprehensive tests covering all functionality
• Tests for banner creation and styling
• Tests for dismissal behavior
• Tests for event emission
• Tests for lifecycle management
• All tests passing with jsdom environment

Configuration

banner: {
  position: 'top' | 'bottom',  // Default: 'top'
  dismissable: boolean,         // Default: true
  zIndex: number                // Default: 10000
}

API

• banner.show(experience) - Show a banner
• banner.remove() - Remove active banner
• banner.isShowing() - Check if banner is active

Closes #7