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/session/ForkTreeModal.tsx

@@ -83,7 +83,7 @@ export function ForkTreeModal({
     <Modal isOpen={isOpen} onClose={onClose} title="Save Custom Tree?" footer={footer}>
       <div className="space-y-4">
         <div className="flex items-start gap-3 rounded-lg bg-accent/50 p-4">
-          <div className="flex h-10 w-10 flex-shrink-0 items-center justify-center rounded-full bg-accent">
+          <div className="flex h-10 w-10 shrink-0 items-center justify-center rounded-full bg-accent">
             <GitFork className="h-5 w-5 text-foreground" />
           </div>
           <div>
@@ -107,7 +107,7 @@ export function ForkTreeModal({
               placeholder="My Custom Tree"
               className={cn(
                 'w-full rounded-md border border-border bg-card px-3 py-2 text-sm text-foreground',
-                'focus:outline-none focus:border-primary focus:ring-1 focus:ring-primary/20'
+                'focus:outline-hidden focus:border-primary focus:ring-1 focus:ring-primary/20'
               )}
             />
           </div>
@@ -124,7 +124,7 @@ export function ForkTreeModal({
               rows={3}
               className={cn(
                 'w-full rounded-md border border-border bg-card px-3 py-2 text-sm text-foreground',
-                'focus:outline-none focus:border-primary focus:ring-1 focus:ring-primary/20',
+                'focus:outline-hidden focus:border-primary focus:ring-1 focus:ring-primary/20',
                 'resize-none'
               )}
             />