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/editor-ai/SuggestionsTab.tsx

@@ -28,7 +28,7 @@ export function SuggestionsTab({ suggestions }: SuggestionsTabProps) {
         return (
           <div key={s.id} className="rounded-lg border border-border bg-[rgba(255,255,255,0.02)] px-3 py-2">
             <div className="flex items-center justify-between">
-              <span className="font-label text-[0.625rem] uppercase tracking-[0.1em] text-muted-foreground">
+              <span className="font-label text-[0.625rem] uppercase tracking-widest text-muted-foreground">
                 {s.action_type.replace(/_/g, ' ')}
               </span>
               <span className={`flex items-center gap-1 text-xs ${config.color}`}>
@@ -39,7 +39,7 @@ export function SuggestionsTab({ suggestions }: SuggestionsTabProps) {
             {s.target_node_id && (
               <p className="mt-1 text-xs text-muted-foreground truncate">Node: {s.target_node_id}</p>
             )}
-            <p className="mt-0.5 font-label text-[0.625rem] text-[#5a6170]">
+            <p className="mt-0.5 font-label text-[0.625rem] text-brand-text-muted">
               {new Date(s.created_at).toLocaleDateString()}
             </p>
           </div>