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/TreeCanvas.tsx

@@ -64,7 +64,7 @@ interface AddNodePickerProps {
 
 function AddNodePicker({ onSelect, onCancel }: AddNodePickerProps) {
   return (
-    <div className="flex items-center gap-2 rounded-xl border border-dashed border-primary/40 bg-card px-3 py-2 shadow-sm">
+    <div className="flex items-center gap-2 rounded-xl border border-dashed border-primary/40 bg-card px-3 py-2 shadow-xs">
       <span className="text-xs text-muted-foreground shrink-0">Add:</span>
 
       <button
@@ -691,7 +691,7 @@ export function TreeCanvas() {
       style={{
         // Subtle dot grid background
         backgroundImage:
-          'radial-gradient(circle, hsl(var(--border)) 1px, transparent 1px)',
+          'radial-gradient(circle, var(--color-border) 1px, transparent 1px)',
         backgroundSize: '24px 24px',
       }}
     >