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/tree-editor/NodeFormDecision.tsx

@@ -104,7 +104,7 @@ export function NodeFormDecision({ node, onUpdate }: NodeFormDecisionProps) {
           className={cn(
             'mt-1 block w-full rounded-md border px-3 py-2 text-sm',
             'bg-card 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',
             questionError ? 'border-red-400' : 'border-border'
           )}
         />
@@ -126,7 +126,7 @@ export function NodeFormDecision({ node, onUpdate }: NodeFormDecisionProps) {
           className={cn(
             'mt-1 block w-full rounded-md border border-border px-3 py-2 text-sm',
             'bg-background text-foreground placeholder:text-muted-foreground',
-            'focus:border-primary focus:outline-none focus:ring-1 focus:ring-primary'
+            'focus:border-primary focus:outline-hidden focus:ring-1 focus:ring-primary'
           )}
         />
       </div>
@@ -187,7 +187,7 @@ export function NodeFormDecision({ node, onUpdate }: NodeFormDecisionProps) {
                       className={cn(
                         'block w-full rounded-md border px-3 py-2 text-sm',
                         'bg-background text-foreground placeholder:text-muted-foreground',
-                        'focus:border-primary focus:outline-none focus:ring-1 focus:ring-primary',
+                        'focus:border-primary focus:outline-hidden focus:ring-1 focus:ring-primary',
                         optionLabelError ? 'border-red-400' : 'border-border'
                       )}
                     />