[Enhancement] Update DatePicker component design, features, and API

[Luizgomess]

✨ Enhancement: Update DatePicker

📖 Description

Revamp the DatePicker component to align with the latest shadcn/ui design standards. This includes updating the visual design, expanding functionality with new variants and features, and refining the public API for better developer experience.

Goals:

• Align visual design with the latest shadcn/ui reference
• Review and update component variants and styling
• Improve API consistency with ZardUI conventions
• Add missing features or demos if applicable

Philosophy: Keep the API minimal and consistent with ZardUI patterns — signal-based inputs, CVA variants, and mergeClasses for styling.

🎨 References

• shadcn/ui: https://ui.shadcn.com/docs/components/date-picker
• Current implementation: libs/zard/src/lib/shared/components/date-picker/

🎯 Scope

1. Design: Update styles to match current shadcn/ui visuals (colors, spacing, borders, animations)
2. Features: Add any missing variants, sizes, or states available in shadcn/ui
3. API: Review inputs/outputs — ensure naming follows z prefix convention and types are strict
4. Demos: Update demo components to match the exact same examples shown on the shadcn/ui documentation page
5. Documentation: Update doc/overview.md and doc/api.md to reflect changes

✅ Acceptance Criteria

• Visual design matches latest shadcn/ui reference
• All variants and sizes are implemented
• API follows ZardUI conventions (z prefix, signal inputs, OnPush, ViewEncapsulation.None)
• Demos updated with simple → complex progression
• Examples match the ones shown on the shadcn/ui docs page
• Accessible (ARIA attributes, keyboard navigation)
• Supports dark mode
• Unit tests updated/added
• Documentation updated

Labels: enhancement