feat: add EsDropdownCheckbox component

[hroth1994]

❓ Type of change

• 📖 Documentation (updates to the documentation or readme)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

• Adds new EsDropdownCheckbox component for multi-select dropdown with checkboxes
• Supports string arrays and {label, value} object arrays, matching EsDropdownSelect API
• Includes Select All / Clear All actions
• Adds documentation page with examples for basic, disabled, required, and error states

🥼 Testing

• Run make dev and navigate to /molecules/dropdown-checkbox
• Verify dropdown opens/closes on click
• Verify selecting/deselecting checkboxes updates the model value
• Verify Select All and Clear All work correctly
• Verify disabled state prevents interaction
• Verify error state displays error message
• Test with both string and object option formats

🧐 Feedback Requested / Focus Areas

• Looking for any and all feedback!

📝 Checklist

• I have verified accessibility of any new components by:
  • automated check with the WAVE browser extension
  • navigating by keyboard
  • using with a screen reader (e.g. VoiceOver on Safari)
• I have updated any applicable documentation.