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/assistant/ChatSidebar.tsx

@@ -31,7 +31,7 @@ export function ChatSidebar({
       <div className="px-4 py-3 border-b shrink-0" style={{ borderColor: 'var(--glass-border)' }}>
         <button
           onClick={onNewChat}
-          className="w-full flex items-center justify-center gap-2 bg-gradient-brand text-[#101114] font-semibold text-sm rounded-[10px] px-4 py-2.5 hover:opacity-90 active:scale-[0.97] transition-all"
+          className="w-full flex items-center justify-center gap-2 bg-gradient-brand text-brand-dark font-semibold text-sm rounded-[10px] px-4 py-2.5 hover:opacity-90 active:scale-[0.97] transition-all"
         >
           <Plus size={16} />
           New Chat
@@ -42,7 +42,7 @@ export function ChatSidebar({
       <div className="flex-1 overflow-y-auto py-2">
         {pinnedChats.length > 0 && (
           <div className="px-3 mb-1">
-            <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">
               Pinned
             </span>
           </div>