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/editor-ai/NodeSummary.tsx

@@ -29,7 +29,7 @@ export function NodeSummary({ node, flowName, flowType, nodeCount }: NodeSummary
             {flowName || 'Untitled Flow'}
           </span>
         </div>
-        <div className="mt-1 flex items-center gap-3 font-label text-[0.625rem] uppercase tracking-[0.1em] text-muted-foreground">
+        <div className="mt-1 flex items-center gap-3 font-label text-[0.625rem] uppercase tracking-widest text-muted-foreground">
           <span>{flowType || 'flow'}</span>
           {nodeCount !== undefined && <span>{nodeCount} nodes</span>}
         </div>
@@ -44,7 +44,7 @@ export function NodeSummary({ node, flowName, flowType, nodeCount }: NodeSummary
     <div className="border-b px-3 py-2.5" style={{ borderColor: 'var(--glass-border)' }}>
       <div className="flex items-center gap-2">
         <Icon className={`h-3.5 w-3.5 ${colorClass}`} />
-        <span className="font-label text-[0.625rem] uppercase tracking-[0.1em] text-muted-foreground">
+        <span className="font-label text-[0.625rem] uppercase tracking-widest text-muted-foreground">
           {node.type}
         </span>
       </div>