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/AIFixReviewModal.tsx

@@ -39,7 +39,7 @@ export function AIFixReviewModal({ fixes, onApply, onApplyAll, onClose }: AIFixR
   const allHandled = pendingFixes.length === 0
 
   return (
-    <div className="fixed inset-0 z-50 flex items-center justify-center bg-black/80 backdrop-blur-sm p-4">
+    <div className="fixed inset-0 z-50 flex items-center justify-center bg-black/80 backdrop-blur-xs p-4">
       <div className="relative flex h-[80vh] w-full max-w-2xl flex-col bg-card border border-border rounded-2xl shadow-lg">
         {/* Header */}
         <div className="flex items-center justify-between border-b border-border px-6 py-4">
@@ -127,7 +127,7 @@ export function AIFixReviewModal({ fixes, onApply, onApplyAll, onClose }: AIFixR
                     <div className="mt-3 flex gap-2">
                       <button
                         onClick={() => handleApply(fix)}
-                        className="flex items-center gap-1 rounded-md bg-gradient-brand px-3 py-1.5 text-xs font-medium text-white shadow-sm shadow-primary/20 hover:opacity-90"
+                        className="flex items-center gap-1 rounded-md bg-gradient-brand px-3 py-1.5 text-xs font-medium text-white shadow-xs shadow-primary/20 hover:opacity-90"
                       >
                         <Check className="h-3 w-3" />
                         Apply