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/tree-editor/ValidationSummary.tsx

@@ -76,7 +76,7 @@ export function ValidationSummary({ errors, onSelectNode, onFixWithAI, isFixing
               'flex items-center gap-1.5 rounded-md px-3 py-1 text-xs font-medium transition-colors',
               isFixing
                 ? 'bg-primary/10 text-primary cursor-wait'
-                : 'bg-gradient-brand text-white shadow-sm shadow-primary/20 hover:opacity-90'
+                : 'bg-gradient-brand text-white shadow-xs shadow-primary/20 hover:opacity-90'
             )}
           >
             {isFixing ? (
@@ -109,7 +109,7 @@ export function ValidationSummary({ errors, onSelectNode, onFixWithAI, isFixing
                   : 'cursor-default'
               )}
             >
-              <AlertCircle className="mt-0.5 h-4 w-4 flex-shrink-0 text-red-400" />
+              <AlertCircle className="mt-0.5 h-4 w-4 shrink-0 text-red-400" />
               <div className="flex-1">
                 <p className="text-red-400">{error.message}</p>
                 {error.nodeId && (
@@ -133,7 +133,7 @@ export function ValidationSummary({ errors, onSelectNode, onFixWithAI, isFixing
                   : 'cursor-default'
               )}
             >
-              <AlertTriangle className="mt-0.5 h-4 w-4 flex-shrink-0 text-yellow-400" />
+              <AlertTriangle className="mt-0.5 h-4 w-4 shrink-0 text-yellow-400" />
               <div className="flex-1">
                 <p className="text-yellow-400">{warning.message}</p>
                 {warning.nodeId && (