Update Progress Bar component to match Figma design specifications

Summary

This MR updates the Progress Bar component to align with the Figma design specifications as outlined in #2565.

Changes Made

• Border radius: Updated to 9999px for fully rounded corners (matching Figma design)
• Size variants: Added three standardized sizes:
  • xs: 4px height
  • sm: 8px height
  • md: 16px height (default)
• Vue component: Added size prop with proper validation
• Backward compatibility: Maintained existing height prop for custom heights

Implementation Details

• Updated progress_bar.scss with new size classes and border-radius values
• Enhanced progress_bar.vue with size prop and computed classes
• All size specifications match the Figma UI Kit exactly
• Tested successfully in Storybook with all size variants working

Testing

• ✅ Verified rounded corners display correctly
• ✅ Confirmed all size variants (xs, sm, md) render with correct heights
• ✅ Tested in Storybook - component controls work properly
• ✅ Backward compatibility maintained for existing height prop usage

Screenshots

The component now displays with fully rounded corners and supports three standardized sizes as per the Figma design.

Closes #2565