Add error view mode and optional Font Awesome auto-include

[MarcinOrlowski]

Summary

This PR adds two major features to DiscoDevBar:

1. Error view mode - Display helpful messages when configuration issues occur
2. Optional Font Awesome auto-include - Automatically include Font Awesome from CDN when needed

Changes

Error View Mode

• Added error state to DiscoDevBarData with customizable error messages
• Distinct red/orange styling for error banner via disco-devbar-error CSS class
• Shows version and helpful link to GitHub when config file is not found
• Flexible design allows for other error messages in the future

Font Awesome Configuration

• Added optional font_awesome section to .disco-devbar.yaml configuration
• Auto-includes Font Awesome from CDN when enabled
• Configurable version (default: 6.5.1)
• Safe defaults: disabled by default to avoid conflicts with existing installations
• All configuration in one place alongside widgets

Example Configuration

font_awesome:
    enabled: true           # Enable auto-include from CDN (default: false)
    version: '6.5.1'        # Font Awesome version (optional, default: 6.5.1)

widgets:
    left:
        - icon: fa-database
          icon_type: fa
          url: /admin
        - icon: 🚀
          icon_type: text
          url: /home

Documentation

• Updated README with comprehensive Font Awesome setup instructions
• Added widget property table including icon_type parameter
• Explained both auto-include and manual setup options

Test Plan

• Test with Font Awesome auto-include enabled
• Test with Font Awesome auto-include disabled
• Test error view when config file is missing
• Test with custom Font Awesome version
• Verify no conflicts when Font Awesome already exists in project