feat: add comprehensive design system with CSS tokens, dark theme, and brand assets - Closes #19

[Jokay1997]

Summary

Implement a complete design system for EquipChain frontend with CSS custom properties, comprehensive color palette, dark theme support, and brand assets.

Changes

Design Tokens (globals.css)

• Brand colors: Full brand-50 through brand-950 blue palette
• Status colors: success, warning, error, info with light/dark variants
• Surface colors: surface, surface-secondary, surface-tertiary, border variants
• Text colors: text-primary, text-secondary, text-muted, text-inverse
• Typography: font-sans (Geist), font-mono (Geist Mono)
• Shadows: shadow-sm through shadow-xl
• Radii: radius-sm through radius-full

Dark Theme

• Comprehensive dark mode using @media (prefers-color-scheme: dark)
• All surface and text tokens have proper dark variants

Brand Assets

• public/logo.svg — Full logo with icon and text
• public/icon.svg — 32px favicon-style icon
• public/logo-mark.svg — 24px standalone mark

Accessibility

• Focus-visible ring style
• Proper selection color

Documentation

• src/lib/design/DESIGN_TOKENS.md — Complete token reference