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/common/CreateFlowDropdown.tsx

@@ -74,7 +74,7 @@ export function CreateFlowDropdown({
       {showMenu && (
         <>
           <div className="fixed inset-0 z-10" onClick={() => setShowMenu(false)} />
-          <div className="absolute right-0 z-20 mt-1 w-64 rounded-lg border border-border bg-card p-1 shadow-xl backdrop-blur-sm">
+          <div className="absolute right-0 z-20 mt-1 w-64 rounded-lg border border-border bg-card p-1 shadow-xl backdrop-blur-xs">
             {/* Troubleshooting */}
             <Link
               to="/trees/new"