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>

---------

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

frontend/src/components/editor-ai/AIPromptDialog.tsx

@@ -45,7 +45,7 @@ export function AIPromptDialog({
     <div className="fixed inset-0 z-50 flex items-center justify-center">
       {/* Backdrop */}
       <div
-        className="absolute inset-0 bg-black/60 backdrop-blur-sm"
+        className="absolute inset-0 bg-black/60 backdrop-blur-xs"
         onClick={() => !isGenerating && onClose()}
       />
 
@@ -68,7 +68,7 @@ export function AIPromptDialog({
           placeholder={`Example: "A flow for troubleshooting VPN connectivity issues when users can't connect to the corporate network"`}
           rows={4}
           disabled={isGenerating}
-          className="w-full rounded-xl border border-border bg-background px-4 py-3 text-sm text-foreground placeholder:text-muted-foreground focus:border-[rgba(6,182,212,0.3)] focus:outline-none resize-none disabled:opacity-50"
+          className="w-full rounded-xl border border-border bg-background px-4 py-3 text-sm text-foreground placeholder:text-muted-foreground focus:border-[rgba(6,182,212,0.3)] focus:outline-hidden resize-none disabled:opacity-50"
           autoFocus
         />
 
@@ -83,14 +83,14 @@ export function AIPromptDialog({
           <button
             onClick={onClose}
             disabled={isGenerating}
-            className="rounded-[10px] bg-[rgba(255,255,255,0.04)] border border-[rgba(255,255,255,0.06)] px-4 py-2 text-sm text-foreground hover:border-[rgba(255,255,255,0.12)] transition-colors disabled:opacity-50"
+            className="rounded-[10px] bg-[rgba(255,255,255,0.04)] border border-brand-border px-4 py-2 text-sm text-foreground hover:border-[rgba(255,255,255,0.12)] transition-colors disabled:opacity-50"
           >
             Cancel
           </button>
           <button
             onClick={handleGenerate}
             disabled={!prompt.trim() || isGenerating}
-            className="flex items-center gap-2 rounded-[10px] bg-gradient-brand px-4 py-2 text-sm font-semibold text-[#101114] shadow-lg shadow-primary/20 hover:opacity-90 active:scale-[0.97] transition-all disabled:opacity-50"
+            className="flex items-center gap-2 rounded-[10px] bg-gradient-brand px-4 py-2 text-sm font-semibold text-brand-dark shadow-lg shadow-primary/20 hover:opacity-90 active:scale-[0.97] transition-all disabled:opacity-50"
           >
             {isGenerating ? (
               <>