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/step-library/CustomStepModal.tsx

@@ -65,7 +65,7 @@ export function CustomStepModal({ isOpen, onClose, onInsertStep }: CustomStepMod
   }
 
   return (
-    <div className="fixed inset-0 z-50 flex items-end justify-center bg-black/80 backdrop-blur-sm sm:items-center sm:p-4">
+    <div className="fixed inset-0 z-50 flex items-end justify-center bg-black/80 backdrop-blur-xs sm:items-center sm:p-4">
       <div className="relative flex h-[95vh] w-full max-w-full flex-col border border-border bg-card shadow-lg sm:h-[90vh] sm:max-w-4xl sm:rounded-2xl">
         {/* Header */}
         <div className="flex items-center justify-between border-b border-border p-4">
@@ -133,7 +133,7 @@ export function CustomStepModal({ isOpen, onClose, onInsertStep }: CustomStepMod
 
         {/* Loading Overlay */}
         {isSubmitting && (
-          <div className="absolute inset-0 flex items-center justify-center bg-black/80 backdrop-blur-sm">
+          <div className="absolute inset-0 flex items-center justify-center bg-black/80 backdrop-blur-xs">
             <div className="flex flex-col items-center gap-3">
               <Spinner className="border-t-foreground" />
               <p className="text-sm text-muted-foreground">Creating step...</p>