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>

frontend/src/components/session/StepRatingModal.tsx

@@ -77,7 +77,7 @@ export function StepRatingModal({
   const getRating = (stepId: string) => ratings.get(stepId)
 
   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="bg-card border border-border w-full max-w-2xl max-h-[90vh] flex flex-col rounded-2xl shadow-lg">
         {/* Header */}
         <div className="flex items-center justify-between border-b border-border px-6 py-4">
@@ -174,7 +174,7 @@ export function StepRatingModal({
                       className={cn(
                         'w-full rounded-md border border-border bg-card px-3 py-2 text-sm text-foreground',
                         'placeholder:text-muted-foreground',
-                        'focus:border-primary focus:outline-none focus:ring-1 focus:ring-primary/20',
+                        'focus:border-primary focus:outline-hidden focus:ring-1 focus:ring-primary/20',
                         'disabled:opacity-50'
                       )}
                     />