Add Accessibility (a11y) Improvements

[gboigwe]

The app lacks proper accessibility features for screen readers and keyboard navigation.

Goal:

• Add ARIA labels to all interactive elements
• Implement keyboard navigation for modals and dropdowns
• Add focus management for modal dialogs
• Ensure proper heading hierarchy
• Add skip-to-content links

Components to update:

• ConnectButton
• NetworkSwitcher
• Event modals
• Navigation menus
• Form inputs