chore: Tailwind CSS v3 → v4 migration (#99)

* chore: run Tailwind v4 upgrade tool (Phase 1)

- Upgraded tailwindcss v3 → v4.2.1, postcss plugin to @tailwindcss/postcss
- Deleted tailwind.config.js, migrated theme to CSS @theme block in index.css
- Replaced @tailwind directives with @import 'tailwindcss'
- Added @custom-variant dark, @utility blocks for custom utilities
- Updated class names across 128 files (shadow-sm → shadow-xs, etc.)
- Removed autoprefixer (built into v4)
- Added migration plan doc

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* chore: switch from @tailwindcss/postcss to @tailwindcss/vite (Phase 2)

- Replaced @tailwindcss/postcss with @tailwindcss/vite plugin
- Deleted postcss.config.js (no longer needed)
- Tailwind now runs as a native Vite plugin for faster HMR

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* refactor: convert to OKLCH colors, move keyframes into @theme (Phase 3-4)

- Replaced all HSL color indirection with direct OKLCH values in @theme
- Moved all keyframes inside @theme block (v4 pattern)
- Eliminated hsl(var(--x)) double-indirection across 17 component files
- Replaced hsl() inline styles with var(--color-*) theme references
- Cleaned up redundant rdp-* utility blocks
- Fixed @custom-variant dark syntax to use :where()
- Added sidebar/glass/shadow vars as OKLCH in :root

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/components/ui/Button.tsx

@@ -3,14 +3,14 @@ import { cn } from '@/lib/utils'
 import { Spinner } from '@/components/common/Spinner'
 
 const buttonVariants = cva(
-  'inline-flex items-center justify-center gap-2 font-medium transition-all duration-200 focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-primary/30 focus-visible:ring-offset-2 focus-visible:ring-offset-background disabled:pointer-events-none disabled:opacity-50 active:scale-[0.97]',
+  'inline-flex items-center justify-center gap-2 font-medium transition-all duration-200 focus-visible:outline-hidden focus-visible:ring-2 focus-visible:ring-primary/30 focus-visible:ring-offset-2 focus-visible:ring-offset-background disabled:pointer-events-none disabled:opacity-50 active:scale-[0.97]',
   {
     variants: {
       variant: {
         primary:
-          'bg-gradient-brand text-[#101114] font-semibold shadow-lg shadow-primary/20 hover:opacity-90',
+          'bg-gradient-brand text-brand-dark font-semibold shadow-lg shadow-primary/20 hover:opacity-90',
         secondary:
-          'bg-[rgba(255,255,255,0.04)] border border-[rgba(255,255,255,0.06)] text-foreground hover:border-[rgba(255,255,255,0.12)] hover:bg-[rgba(255,255,255,0.06)]',
+          'bg-[rgba(255,255,255,0.04)] border border-brand-border text-foreground hover:border-[rgba(255,255,255,0.12)] hover:bg-brand-border',
         destructive:
           'bg-red-400/10 text-red-400 border border-red-400/20 hover:bg-red-400/20',
         ghost: