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/layout/CommandPalette.tsx

@@ -124,10 +124,10 @@ export function CommandPalette({ open, onClose }: CommandPaletteProps) {
   if (!open) return null
 
   return (
-    <div className="fixed inset-0 z-[100] flex items-start justify-center pt-[20vh]">
+    <div className="fixed inset-0 z-100 flex items-start justify-center pt-[20vh]">
       {/* Backdrop */}
       <div
-        className="absolute inset-0 bg-black/60 backdrop-blur-sm animate-fade-in"
+        className="absolute inset-0 bg-black/60 backdrop-blur-xs animate-fade-in"
         onClick={onClose}
       />
 
@@ -143,7 +143,7 @@ export function CommandPalette({ open, onClose }: CommandPaletteProps) {
             onChange={e => { setQuery(e.target.value); setSelectedIndex(0) }}
             onKeyDown={handleKeyDown}
             placeholder="Search flows, sessions…"
-            className="flex-1 bg-transparent text-sm text-foreground placeholder:text-muted-foreground outline-none"
+            className="flex-1 bg-transparent text-sm text-foreground placeholder:text-muted-foreground outline-hidden"
           />
           <kbd className="rounded border border-border bg-background px-1.5 py-0.5 font-label text-[0.625rem] text-muted-foreground">
             ESC