feat(theme): Modernize v9 theme to create v10 with contemporary design system

[ruanyl]

Summary

This CR introduces the v10 theme, a modernized version of the v9 theme with a contemporary, shadcn-inspired aesthetic. The original v9 theme is preserved and remains available, giving users a choice between the classic v9 design and the modern v10 design.

Preview the changes: ruanyl.github.io/oui/

What Changed

Design Token Updates

• Border Radius: Increased from 4px to 8px (with 6px and 12px variants) for a softer appearance
• Shadows: Implemented softer, layered shadows with reduced opacity for better depth perception
• Typography: Expanded font weight range (300-700) with improved line heights for better readability
• Spacing: Established consistent 16px-based scale across all components
• Colors: Migrated to modern Slate-based palette with improved contrast and accessibility

Color System Modernization

Light Theme

• Primary: #0268BC → #2563EB (more vibrant blue)
• Success: #0F7B68 → #10B981 (distinct green)
• Neutrals: Blue-tinted → Slate-based with warm undertones
• Background: #F0F2F4 → #F8FAFC (softer, cleaner)

Dark Theme

• Primary: #0097D1 → #3B82F6 (brighter, more visible)
• Text: #959BA2 → #CBD5E1 (significantly improved contrast)
• Neutrals: Cold grays → Refined slate-based palette
• Background: #02020E → #020617 (richer depth)

Component Enhancements

• Buttons: Modern border radius with smooth transitions
• Forms: Rounded inputs with enhanced focus states
• Panels/Cards: Elevated appearance with refined shadows
• Modals: Larger radius for visual prominence
• SideNav: Subtle background highlight with accent bar for selected items
• Badges: Rounded corners, improved spacing, semibold typography
• CallOuts: Rounded corners with subtle borders and left accent bars
• FilterGroups: Connected buttons with rounded ends and modern selected states
• DataGrid: Rounded container, enhanced headers, optimized padding, smooth hover transitions
• Toast: Rounded corners, left accent bar, semibold titles, always-visible close button

Backward Compatibility

✅ No Breaking Changes

• All variable names unchanged
• EUI aliases maintained
• Components work without modification
• Existing v9 theme preserved and available

Issues Resolved

Check List

• New functionality includes testing.
• New functionality has been documented.
• All tests pass
  • yarn lint
  • yarn test-unit
• Update CHANGELOG.md
• Commits are signed per the DCO using --signoff

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