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/FlowCanvasAnswerNode.tsx

@@ -16,7 +16,7 @@ function FlowCanvasAnswerNodeComponent({ data, selected }: NodeProps) {
 
   return (
     <>
-      <Handle type="target" position={Position.Top} className="!bg-border !w-2 !h-2 !border-0" />
+      <Handle type="target" position={Position.Top} className="bg-border! w-2! h-2! border-0!" />
 
       <div
         className={cn(
@@ -61,7 +61,7 @@ function FlowCanvasAnswerNodeComponent({ data, selected }: NodeProps) {
         )}
       </div>
 
-      <Handle type="source" position={Position.Bottom} className="!bg-border !w-2 !h-2 !border-0" />
+      <Handle type="source" position={Position.Bottom} className="bg-border! w-2! h-2! border-0!" />
     </>
   )
 }