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>

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Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/components/tree-editor/FlowCanvas.tsx

@@ -139,14 +139,14 @@ function FlowCanvasInner({ selectedNodeId, onNodeSelect, onSelectAnswerType, onN
         proOptions={{ hideAttribution: true }}
         className="dark bg-accent/30"
       >
-        <Background variant={BackgroundVariant.Dots} gap={20} size={1.5} color="hsl(var(--muted-foreground) / 0.25)" />
-        <Controls showInteractive={false} className="!bg-card !border-border !shadow-lg" />
+        <Background variant={BackgroundVariant.Dots} gap={20} size={1.5} color="oklch(0.63 0.02 260 / 0.25)" />
+        <Controls showInteractive={false} className="bg-card! border-border! shadow-lg!" />
         {minimapVisible && (
           <MiniMap
             pannable
             zoomable
             nodeColor={minimapNodeColor}
-            className="!bg-card !border-border"
+            className="bg-card! border-border!"
             nodeStrokeWidth={2}
           />
         )}