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/ChatTab.tsx

@@ -50,7 +50,7 @@ export function ChatTab({ messages, input, onInputChange, onSend, isLoading }: C
               className={`max-w-[85%] rounded-xl px-3.5 py-2.5 text-[0.8125rem] leading-relaxed ${
                 msg.role === 'user'
                   ? 'bg-primary/15 text-foreground'
-                  : 'bg-[rgba(255,255,255,0.04)] text-foreground border border-[rgba(255,255,255,0.06)]'
+                  : 'bg-[rgba(255,255,255,0.04)] text-foreground border border-brand-border'
               }`}
             >
               <MarkdownContent content={msg.content} className="text-[0.8125rem] leading-relaxed" />
@@ -59,7 +59,7 @@ export function ChatTab({ messages, input, onInputChange, onSend, isLoading }: C
         ))}
         {isLoading && (
           <div className="flex justify-start">
-            <div className="bg-[rgba(255,255,255,0.04)] border border-[rgba(255,255,255,0.06)] rounded-xl px-3.5 py-2.5">
+            <div className="bg-[rgba(255,255,255,0.04)] border border-brand-border rounded-xl px-3.5 py-2.5">
               <Loader2 size={16} className="animate-spin text-primary" />
             </div>
           </div>
@@ -77,14 +77,14 @@ export function ChatTab({ messages, input, onInputChange, onSend, isLoading }: C
             onKeyDown={handleKeyDown}
             placeholder="Ask AI to help..."
             rows={1}
-            className="flex-1 resize-none rounded-xl border bg-card text-foreground text-[0.8125rem] placeholder:text-muted-foreground px-3.5 py-2.5 focus:outline-none focus:border-[rgba(6,182,212,0.3)]"
+            className="flex-1 resize-none rounded-xl border bg-card text-foreground text-[0.8125rem] placeholder:text-muted-foreground px-3.5 py-2.5 focus:outline-hidden focus:border-[rgba(6,182,212,0.3)]"
             style={{ borderColor: 'var(--glass-border)' }}
             disabled={isLoading}
           />
           <button
             onClick={onSend}
             disabled={!input.trim() || isLoading}
-            className="bg-gradient-brand text-[#101114] p-2.5 rounded-xl hover:opacity-90 active:scale-[0.97] transition-all disabled:opacity-40"
+            className="bg-gradient-brand text-brand-dark p-2.5 rounded-xl hover:opacity-90 active:scale-[0.97] transition-all disabled:opacity-40"
           >
             <Send size={16} />
           </button>